Merge pull request #104 from NESCent/language-trees

Basic support for language trees
Set many=True on LanguageTreeSerializer
adding __unicode__ to source
Merge pull request #123 from SimonGreenhill/master

adding __unicode__ to source
testing
Merge remote-tracking branch 'upstream/master'
Rewrite README to include installation and dev documentation
Merge pull request #129 from NESCent/install-docs

Rewrite README to include installation and dev documentation
changing Society.source to foreign key on Source model
working around a typecast issue
adding sources file for loader
adding source field to EnvironmentalValue
adding source field to EnvironmentalValue
adding source to more models
updating tests
adding sources to environmental
making an empty isocode return None too
adding some rudimentary caching to go nice on the database
adding source to binford data
updating source info in EA societies