Hack's version of Pimple

This is Pimple version written in hack. 

The API is mostly the same except the next changes:
- offsetSet() and offsetUnset(), the return `$this` as requirement of the interface `ArrayAccess<Tk, Tv>`
- keys(), returns a Vector instead an array
-  protected(), factory() and extends() now dont throw InvalidArgumentException, now is Fatal error due to the types.

Due to this issue https://github.com/hhvm/hack-hhvm-docs/issues/192, the type hinting not exists at offsetSet, offsetUnset, offsetGet and offsetExists functions

The tests are the same with support for the behaviour changes: https://github.com/mcuadros/pimple-hack/commit/687dd878ad30b2715720ed84174969c916424455

I must upload a copy of it, with this changes at the hack folder?

This PR was made following the @fabpot requirements 

Thanks!