Replace absolute file path with base filename

Updating a gist with an absolute filepath yields the error "contents
can't be stored in subdirectories".  Instead of using the full path to
the file, we will instead use the base filename.
don't set buffer-file-name

after all, we're not visiting a file
Merge pull request #49 from keelerm84/bug/invalid-slot-type

gist-add-buffer: Invalid Slot Type