Allow custom path for thumbnail creation

1. Added settings to generate thumbnails in user defined basedirs
Make BASEDIR settings inside THUMBNAILS key
Added default basedir
Merge pull request #13 from SeiryuZ/thumbnail-folder

Allow custom path for thumbnail creation
Merge branch 'master' of github.com:ui/django-thumbnails into processors

Conflicts:
	thumbnails/conf.py
	thumbnails/tests/fields.py