Add the `tar` module to `vendor`.
Remove the Closure Compiler from `vendor`.
`build.js`: Lazily download the Closure Compiler.

The `getCompiler()` helper attempts to download the latest version of
the Compiler from Google's servers, stores the `ETag` of the requested
bundle in `vendor/closure-compiler-etag.txt`, and extracts the `.jar`
from the bundle into `vendor/closure-compiler.jar`.

The cached `ETag` is presented in the `If-None-Match` header for all
subsequent requests, ensuring that the Compiler is only re-downloaded
when a new version is available. If the latest version cannot be
downloaded, the builder will use a previously-downloaded copy.

Addresses #54.
`build.js`: Fix `getCompiler()` for offline use.

`getCompiler()` should not wait for `saveETag()` if a connection
error occurs and the `response` event is never fired.