Fix FormBuilder error when no Sprockets installed.

Sprockets was built-in since Rails 3.1.
Do not include Sprockets helpers if it's not defined.
Merge pull request #40 from bootleq/sprockets

Fix FormBuilder error when no Sprockets installed.
With this Rails 4 can be used too without the set_table_name error.
Adding translation messages that are missing in the README.rdoc
Merge pull request #43 from arthurmolina/master

Compatibilty with Rails 4
merge conflict Sprockets
merge conflict: set table name