Per-browser rpc.js [#3]

description:
Cleaned up UserAgent class and UserAgent.Browser.OTHER handling. Renamed "Smart" RpcJsFeatureLoader to "BrowserSpecific". A few other small cleaups. Open: whether to serve JS as Cache-Control private always for simplicity.

[description, update #2]
Updated w/ private Cache-Control headers to prevent CDN proxying issues.

[original description]
Implementation of JsFeatureLoader that loads only the rpc.js transport
code
needed by the target browser.

To use this, a concrete implementation of UserAgent.Parser must be
provided and
bound. In addition, JsFeatureLoader must be bound to implementation
SmartRpcJsFeatureLoader.