Tidy up and CS fixes

- Renamed $_elements to $elements
- Added new line before return statements
- Returned early where possible
- Fixed usage of constant
- Removed duplicated code
- Removed unnecessary return statement in offsetSet
OMG. Thanks, I see the point. Will change it back then.

I wanted to ask you about the doctrine coding standards with regarding to the 'extra' spaces in the conditional structures and when using the logical operator `!`. I've found [this document](http://doctrine1.readthedocs.org/en/latest/en/manual/coding-standards.html#control-statements) but I can't seem to find anything updated. Is it still valid or should it be ignored and just follow PSR-2?