Adds a check in the `rake clean` process for nib/xib existence.

Fixes #78 by checking for the existence of a xib when attempting to clean a nib file. If a xib file exists, delete the nib; otherwise, delete it.
Merge pull request #98 from imkmf/78-nib-xib-rake-clean-edits

Adds a check in the `rake clean` process for nib/xib existence.