Color functions: all blending functions
Color functions: tint, shade
Color functions: fix hue, saturation, lightness rounding.

I didn't notice that they round them in less.js! Now the color functions 
test-cases all pass.
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?
Functions test: tweak css output so tests pass, results were equivalent
and arguably more correct? ;-)
Functions: implement pi()

This function doesn't take parameters, which was not supported in the
grammar. I have changed the grammar (added a ?) and modified TermBuilder
to recognise the change and created EmptyExpression to represent the
lack of parameters.

The pi test is now returned to functions.less.
Merge branch 'color-functions'