first attempt at a travis config

This just does a build+test. I'll try the install scripts (for #58) in a
following commit.

.travis.yml

@@ -0,0 +1,13 @@
+language: rust
+rust:
+  - stable
+  - nightly
+  - 1.17
+matrix:
+  allow_failures:
+    - rust: nightly
+cache: cargo
+script:
+  - cargo build --verbose --all
+  - cargo test --verbose --all
+  - 'if [ $TRAVIS_RUST_VERSION = nightly ]; then cargo bench --verbose --all; fi'