Fix for codeplex-1357

This change allows users to configure the casing behavior of simple
membership provider. The simple membership provider will by default
generate a database query that normalizes the case of usernames on the
database side.

This comes with the side effect of obviating any index that the user may
have configured for the user name column.

The fix is to make this behavior configurable. With the new option, it
will be possible to turn off casing normalization, and allow the database
to handle it specific to its collation.