fadein and fadeout add and substract from the alpha value rather than
multiply.

Example:

fadein(rgba(0,0,0,.5), 50%) = rgba(0,0,0,1)

before it would have been rgba(0,0,0,.75).
Updated management functions in Selector.

Added find(). split() now returns a list of Selector objects instead
of TokenList objects.
Changed TokenList::toString() to return a string instead of a pointer.
Renamed ParameterMixin to Mixin.
Bugfix for issue #19: Shortening color hashes.

#AABBCC was being converted to #ABB instead of #ABC.
Refactoring

- Changed function arguments from pointers to references wherever
  possible.
- Added const to function arguments that aren't changed in the
  function.
- Removed ExtendCssWriter and added Extension. Extensions are a little
  too complicated to handle in a CssWriter subclass so they are take
  care off in LessStylesheet::process() now.
- Added ProcessingContext to contain the state during processing
  (variables and mixin stack).
- Separated scopes from ValueProcessor into ValueScope.
Made Token an extension of std::string.

Removed str member.
Changed references to Token.str to the Token itself.
Changed references to Token.str to the Token itself.
Updated process() call in LessMediaQuery::process() to

include context.
Fixed LessRuleset::putArguments() to work with LessSelector::getDefault()

LessSelector::getDefault() now returns an empty TokenList instead of NULL.
Fixed LessSelector::matchArguments()

It returned false when it did find a default value.
Added ProcessingContext argument to LessStylesheet::process() function.
Updated Mixin::parseArgument() to skip over delimiters.
Added ProcessingContext to lessc.
Fixed bug in LessSelector that caused duplicate parts in the selector.
Updated MediaQueryRuleset::process() so it overloads LessRuleset::process().
Fixed MediaQueryRuleset output.

Inside another media query it was putting the "and" in the wrong
place:

@media print {
   @media screen {
   }
}

created:
and@media print screen

instead of:
@media print and screen
Updated Selector::match() so that it can match against selectors with commas

'.a' now matches '.a, .b' and '.b, .a'
Fixed extensions.