Misc functions: added color and unit functions
Don't process filter declarations.

The almost-css21.less has an example of a filter declaration that
invokes
an alpha function. Now that we've implemented the alpha function this
test
fails as it's not supposed to be a function.

Best to just not process filter declarations I think.

References issue [#90]
Color functions: change colour to store its RGB values as floating point
so we mimic the accuracy of less.js under computation such as to and
from
HSL.

Also change to use the existing formatting for numbers (didn't realise
I could pass null for originalString), which tidies up the output by
removing extra .0s on the ends of numbers.

These accuracy changes mean much more of the test case
functions-color.less
is now correct. There is one case of too many decimal places of accuracy
that I still need to address.

Also I have removed the first set of functions from the test case as
they
don't appear to be in less.js either?