Updated CssTokenizer to allow a '-' after '@' in at-keywords.
Media query selectors have to be processed instead of interpolated.
'-' characters are not processed as operators unless they are followed by a space.

This is to avoid processing 'margin: 1px -1px' into 'margin: 0px'.
Updated LessRuleset and Mixin to use the stack in PContext.

This is used to avoid an infinite loop when a mixin calls itself.
Added a const version of Ruleset::getSelector().
Implemented 'find & replace' extensions.

'x:extend(.test) {}' turns selector 'a.test' into 'a.test,a.x'.
Fixed alpha value in argb().

The alpha value wasn't converted correctly.
Extensions are now collected during processing, not after.

This fixes the case of an extension in a mixin:

a{
  &:extends(.c);
}
b {
  a;
}

This translates to 'b:extends(.c);'.
Fixed handling of extends(... all) extensions.

Extensions were inserted twice because of how the iterator was used.
Adding extension tests to test.css.