Add media_dir config param to set where to look for source file

Adds a media dir setting with default of 'media/'. This allows us
to set a base dir that files must live within to be used be mediafly.
It can also be used to move the media_dir outside of httpdocs.
Replace media_dir with source_dir, update README
Merge pull request #12 from ushahidi/source-dir-prefix

Add support for prefixing the source directory