Add Git crawler, and implementations for GitHub, Gitorious, Bitbucket.

[sevko]

Robustness

• [x] Add basic Git indexer.
• [x] Add threading to indexer and GitHub crawler.
• [x] Add threaded cloner.

  Framework crawlers

• [x] Add GitHub crawler.
• [x] Add Gitorious crawler.
• [x] Add Bitbucket crawler.

  Enhancements / Bugs

• [x] Fix directory-deletion failure bug.
• [x] Add file-extension regex rules.
• [x] Add more boilerplate-file regex rules.

[sevko]

A generalized Git indexer, and crawlers for GitHub and Bitbucket are in place; because Gitorious doesn't have a particularly capable API, and since we don't require any more additional Codelet harvesting capacity, closing for now.