Allow more flexibility in applying individual patches #20

I'm using patch in an app that needs to accept patch files and apply them 
safely contained within a certain directory. Namely, it needs to vet each file 
that is to be modified and fail if a patch attempts to modify any file outside 
the base directory that the patch is to be applied to - this could be due to 
symlinks, relative or absolute paths.

To enable this, I've split up the `apply` function into two parts:

patchset.file_patches()
 - returns a generator of (filename, patch) pairs - note that this is the filename that will be modified (so it may be either the source or target)

patchset.apply_patch(filename, i, p):
 - applies an inidivual patch to the given filename. I've left the variable names as-is for the sake of a minimal patch, but `fileno` and `patch` may be better names.

This makes `patchset.apply` a very simple piece of code that just hooks these 
two functions together. My app can now do the same thing, but insert some 
file-checking code before each patch gets applied. Previously it would have to 
duplicate the logic in `apply`, which is quite a huge undertaking.

Other bits:

 - I did have to move the `errors` count into a field on PatchSet, because there's no longer any shared scope between `apply` and `pply_patch`.

 - fixed `xstrip` function which never actually returned anything but `None`.

 - Added "normalize_filenames" params to `parse` (and init) function defaulting to True, to optionally prevent path normalization (which just confuses the matter for my application).


`patch` is the actual patch - I'm also attaching `patch_whitespace_indifferent` 
just for your information (since the change alters a lot of whitespace).