Migrate to govendor to avoid limitations of godep

- over the course of a project history every maintainer needs to update
  its dependency packages, the problem essentially with godep is manipulating
  GOPATH - this manipulation leads to static objects created at different locations
  which end up conflicting with the overall functionality of golang.

  This also leads to broken builds. There is no easier way out of this other than
  asking developers to do 'godep restore' all the time. Which perhaps as a practice
  doesn't sound like a clean solution. On the other hand 'godep restore' has its own
  set of problems.

- govendor is a right tool but a stop gap tool until we wait for golangs official
  1.5 version which fixes this vendoring issue once and for all.

- govendor provides consistency in terms of how import paths should be handled unlike
  manipulation GOPATH.

  This has advantages
    - no more compiled objects being referenced in GOPATH and build time GOPATH
      manging which leads to conflicts.
    - proper import paths referencing the exact package a project is dependent on.

 govendor is simple and provides the minimal necessary tooling to achieve this.

 For now this is the right solution.

CONTRIBUTING.md

@@ -18,8 +18,7 @@ $ make
 Checking if proper environment variables are set.. Done
 ...
 Checking dependencies for Minio.. Done
-Installed godep
-Installed cover
+Installed govet
 Building Libraries
 ...
 ...
@@ -37,8 +36,7 @@ $ make
 Checking if proper environment variables are set.. Done
 ...
 Checking dependencies for Minio.. Done
-Installed godep
-Installed cover
+Installed govet
 Building Libraries
 ...
 ```
@@ -52,20 +50,17 @@ Building Libraries
     - Push to the branch (git push origin my-new-feature)
     - Create new Pull Request
 
-* If you have additional dependencies for ``Minio``, ``Minio`` manages its depedencies using [godep](https://github.com/tools/godep)
+* If you have additional dependencies for ``Minio``, ``Minio`` manages its depedencies using [govendor](https://github.com/kardianos/govendor)
     - Run `go get foo/bar`
     - Edit your code to import foo/bar
-    - Run `make save` from top-level directory (or `godep restore && godep save ./...`).
+    - Run `govendor add foo/bar` from top-level directory
+
 * When you're ready to create a pull request, be sure to:
     - Have test cases for the new code. If you have questions about how to do it, please ask in your pull request.
-    - Run `go fmt
-    - Run `golint`
-        ```
-	$ go get github.com/golang/lint/golint
-	$ golint ./...
-	```
+    - Run `make verifiers`
     - Squash your commits into a single commit. `git rebase -i`. It's okay to force update your pull request.
     - Make sure `go test -race ./...` and `go build` completes.
+
 * Read [Effective Go](https://github.com/golang/go/wiki/CodeReviewComments) article from Golang project
-    - `Minio` project is strictly conformant with Golang style
+    - `Minio` project is fully conformant with Golang style
     - if you happen to observe offending code, please feel free to send a pull request