Refactor BootStrapper to delegate fetching Galaxy source to DownloadProperties object.

Will enable refactoring toward alternative 'downloaders'.
Refactor a Downloader interface out of Download properties.

Again, this will enable the implementation of a direct github zip download.
Implement github downloader.

Download galaxy-central as a zip file right from @jmchilton's github fork of Galaxy.
Improved error messages.
Clean up, extend, and clarify the public 'interface' for DownloadProperties.