Add additional methods of specifying the 'select' portion of a query.

This commit adds more ways of specifying selects:

-) You can now pass in a []string.  This is mostly for convenience,
since you may want to dynamically create a list of fields to be
selected.

-) You can now use variables.  This is important because a select
could take user input.  For example, finding a MAX between a record
and a given number could be easily done using select, and then
you don't have to process anything in backend logic.  This is also
necessary to use postgres text search capabilities (which actaully
play nicely with the rest of gorm).

-) You can now chain select calls.  This could be useful in
conjunction with gorm's scopes functionality.