Added support for min, max, abs, convert and enhanced others

[ghost]

trafficstars

Added built in min, max, abs and convert functions.

Enhanced mod and pow so that they return the unit of the first parameter as per the less function reference: http://lesscss.org/functions/#math-functions-mod

All new features are supported by unit tests.

[glensc]

added in https://github.com/MarcusSchwarz/lesserphp/releases/tag/v0.5.1