Updated Selector to support cloning selectors and fixed addPrefix() so
it doesn't crash when adding a prefix to an empty selector.

Added Selector::trim() to make it easy to remove unnecessary
whitespace.
Fixed Ruleset::clone() so it clones its selector instead of copying
the pointer.
Added a ParameterMixin class that holds information to a Parametric
ruleset call and added support for parametric mixins inside parametric
rulesets.

Parametric ruleset:
test(@a) {
       x: @a
}

Parametric mixin:

test(1px);

Nested parametric mixin:

test1(@a) {
          x: @a
}
test2(@b) {
          test1(@b)
}
Updated LessParser to support Parametric mixins inside Parametric rulesets.