Added a function that find the categories defined in the bib file.

	  * gitbib.py: Here.
Categories now handled while adding.

		   * gitbib.py: Here.
Added category editing.

	  * gitbib.py: Here.
Fix bug that occured when no file was given.

	* bibtexentry.py: Here.
Cosmetic changes.

		 * bibtexentry.py: Add a line break between entries.
		 * gitbib.py: Remove a buggy and useless print.
Added a sanity check for the fields.

	  * bibtexentry.py: Sanity check for each type of entry.
	  * gitbib.py: Call sanity check after add or edit.