Removed case conversion on custom validator extensions

Reference laravel/framework#2482

Rule name was being converted between snake_case and StudlyCaps throughout the code unnecessarily when using Validate::extend(). This forced the user to define custom validation rule names in snake_case only, otherwise it would throw an exception. Removed all case conversion throughout so user can define rule names as desired.

The only reason the case conversion seemed to make any sense is that the rule name gets concatenated into a magic method and called in validate() (line 257). Didn't seem that important to have proper method camelCasing just for this. It also seemed simpler to remove the conversions than adding a bunch more when the rule was added, and also throughout the validation message handling.