Proper rgba() function alpha output

The alpha value can either be a number in the range 0-1 or a percentage
(0-100%). Alpha values greater than 1 are simply dropped. Compare with
the rgba() function definition at http://lesscss.org/#reference.
Add argb() function

Used by bootstrap3. Quote from the lesscss.org documentation:

Creates a hex representation of a color in #AARRGGBB format (NOT
Android development.