Merge pull request #34 from glebm/user_class

Use Thredded.user_class instead of User
[JRO] Create more informative initializer

Add a generator that will install an initializer with the available
config options.
[JRO] Update README and initializer

The README must reflect the latest information on how to install the gem
so the updates to the install generator, and available configurations,
are added to the doc as of now.

The initializer has been updated with several further bits that are
customizable.
Case-insensitive string matching for MySQL and PG

On MySQL text columns are case-insensitive.
On PostgreSQL, they are case-sensitive by default.

Also adds Thredded.user_name_column config