Preserve trailing slash in signature base

This patch changes the library to preserve the trailing slash when computing
the signature base string.

-[NSURL path] function always excludes a trailing slash. Since the oauth specification
makes no mention of excluding a trailing slash, it seems correct to preserve it. At least
one python server-side library will not accept requests without this patch.
Update ab_actualPath to use CFURLCopyPath

This should be a little more reliable
Merge pull request #5 from socialize/allow-trailing-slash

Preserve trailing slash in signature base