v0.5.1

This release is based on leafo/lessphp v0.5.0
It adds php7-compatibility and includes some of the pull requests for the original leafo/lessphp-repository

These are:
leafo#523
leafo#540
leafo#564
leafo#566
leafo#578
leafo#584
leafo#590
leafo#592
leafo#601
leafo#603
leafo#604
leafo#605
leafo#607
leafo#610
leafo#616
leafo#619