Make Str::camel use lower camelCase instead of upper CamelCase

I believe this is the default, not only for camelCase in general, but also in the framework itself.



Sources:
* [PSR-1](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-1-basic-coding-standard.md)
* [msdn](http://msdn.microsoft.com/en-us/library/x2dbyw72\(v=vs.71\).aspx)
* [Wikipedia](http://en.wikipedia.org/wiki/CamelCase)