Added support for pre-download over Mercurial.

Enables pods to be included in Podfiles by specifying the root of a Mercurial repository containing a podspec. Works just like the existing :git and :svn support.

For example, can now include the following line in a Podfile:

pod 'VeriJSON', { :hg => 'https://dcutting@bitbucket.org/dcutting/verijson', :revision => '0.1.0' }