attemp1 to make TravisCI help us out

If this works properly it should automatically rebuild and deploy the `docs/` directory upon each commit to master (because we'll probably forget some of the time).

.travis.yml

@@ -0,0 +1,25 @@
+language: rust
+sudo: false
+
+cache:
+  - cargo
+
+rust:
+  - stable
+
+before_script:
+  - (test -x $HOME/.cargo/bin/cargo-install-update || cargo install cargo-update)
+  - (test -x $HOME/.cargo/bin/mdbook || cargo install --vers "^0.1" mdbook)
+  - cargo install-update -a
+
+script:
+  - cd book && mdbook build && mdbook test
+
+deploy:
+  provider: pages
+  skip-cleanup: true
+  github-token: $GITHUB_TOKEN
+  local-dir: docs
+  keep-history: false
+  on:
+    branch: master