Homogenize path-separator in staging_parse

The staging_parse() function manipulates input using Ruby libraries and
path logic. However, the Ruby URI library expects that (even on Windows)
paths will use "/" as a separator. Rather than special-case everything,
resolve this inconsistency regarding Windows paths by enforcing that
backslashes will simply be converted to forward-slashes before trying to
operate on them with staging_parse(). Otherwise, using a source or
target in the form of a Windows path with backslashes will cause the
staging_parse() function to abort.
Merge pull request #25 from reidmv/windows_staging_parse

Homogenize path-separator in staging_parse