Fix references to non-existant variables in rbt patch

rbt patch in RBTools 0.5 is pretty broken - it was using attributes
which didn't exist (Patch.repository_info and Patch.tool instead of
the local variables that it creates).

I was getting the following error messages before these fixes (not all
at once)

CRITICAL:root:'Patch' object has no attribute 'repository_info'
CRITICAL:root:'Patch' object has no attribute 'tool'
CRITICAL:root:'NoneType' object has no attribute 'get_diffs'

Reviewed at http://reviews.reviewboard.org/r/4022/
Add a method for retreiving resources from a path

The RBClient now has a 'get_path' method which will accept a path and
retreive the API resource at that path. For example a path of
'/review-requests/123' would retreive the resource at
'http://example.com/api/review-requests/123'

A 'ParseError' Exception has also been added for commands. If there is
an error parsing the command arguments or options, the command should
raise a ParseError.