Added option to overwrite existing stored credentials.

Added an option to django_orm.Storage locked_put method to allow for
overwriting an existing credential record rather than the creation of a
new one.
Wrote doc string for new storage parameter.

Added proper doctoring for the overwrite parameter in Django ORM
storage and updated an unused variable.
Merge branch 'master' of git://github.com/google/oauth2client
Merge pull request #2 from lraccomando/master

Added option to overwrite existing stored credentials.