Switch to 'request' module for HTTP

This supports proxies and streaming.
Add --proxy option for xwalk_android_dl
Add note about --proxy option to docs
Update tests for Downloader
Set proxy from environment if available
Correctly set proxy for VersionsFetcher

The VersionsFetcher instance in xwalk-android-dl.js was a
"vanilla" instance with no proxy setting.

This passes the HttpClient into the constructor so the
VersionsFetcher instance will use a proxy if provided.
Merge pull request #37 from crosswalk-project/proxy-27

Support proxies in downloader