Add new test-cases for functions from less.js
Math functions: update round function to support two arguments (second
being dp)
Less4j cannot parse the pi: declaration and fails with an error.
Functions: implement remaining math functions
Functions: convert function

I have added a convertTo function to NumberExpression, based on the
implementation
in less.js.
Functions: added type functions, eg. iscolor etc
Functions: extract
Functions: remove add and increment functions and their tests, also 
remove _color("evil red").

These apparently are just test functions, perhaps to test custom
functions?