Add Support for Manta Object Storage as a Gateway (#5025)

Manta is an Object Storage by [Joyent](https://www.joyent.com/)

This PR adds initial support for Manta. It is intended as non-production 
ready so that feedback can be obtained.

vendor/github.com/joyent/triton-go/Gopkg.toml

@@ -0,0 +1,42 @@
+
+# Gopkg.toml example
+#
+# Refer to https://github.com/golang/dep/blob/master/docs/Gopkg.toml.md
+# for detailed Gopkg.toml documentation.
+#
+# required = ["github.com/user/thing/cmd/thing"]
+# ignored = ["github.com/user/project/pkgX", "bitbucket.org/user/project/pkgA/pkgY"]
+#
+# [[constraint]]
+#   name = "github.com/user/project"
+#   version = "1.0.0"
+#
+# [[constraint]]
+#   name = "github.com/user/project2"
+#   branch = "dev"
+#   source = "github.com/myfork/project2"
+#
+# [[override]]
+#  name = "github.com/x/y"
+#  version = "2.4.0"
+
+
+[[constraint]]
+  branch = "master"
+  name = "github.com/abdullin/seq"
+
+[[constraint]]
+  name = "github.com/davecgh/go-spew"
+  version = "1.1.0"
+
+[[constraint]]
+  branch = "master"
+  name = "github.com/hashicorp/errwrap"
+
+[[constraint]]
+  branch = "master"
+  name = "github.com/sean-/seed"
+
+[[constraint]]
+  branch = "master"
+  name = "golang.org/x/crypto"