Always access Builder via builder() helper

Proposing this as a separate PR in order to remove some of the *black magic* when trying to embed this library.
This simple change makes its behavior way more predictable, i.e. so that whenever you call `builder()`, you will actually *get* a `phake\Builder` instance.

Depends on #20, actual commit is only c414074c5a6bd9d3304c3277f0b3f80460c5a894.