Don't checkout a git branch if not given

"master" is not always the default branch. Just leave it as the default one
that it is cloned from.

Fixes conda/conda#738
Merge branch 'master' into rpath
Add py34 selector variable
Simplify some logic
Extend BuildRoot to track non-DLL files.

(I'm using "DLL" in this context to refer to anything that passes the `is_obj`
test (for Linux, anything that looks like an ELF binary).)
Merge branch 'master' into rpath
Python 3 compatibility issue
Remove incorrect test

See https://github.com/conda/conda-build/pull/111#discussion_r13295629
bools are already ints
isdigit is better than is_int
Clean up some code with defaultdict
Delete debug lines
Simplify a piece of code
Remove redundant piece of code
Remove commented out line
Remove unused imports
Delete unused possible_link_target_names
Use with_metaclass
Fix defaultdict usage
Fix str/bytes issue