Enable multi-Element Mixin Selectors

Previously, dotLess would require that mixins be selectors with single
elements (eg. ".dropdown-menu") since the matching algorithm would try
to match only a single element and then drill down into the selector's
rules for any additional elements. Bootstrap uses multi-element mixin
selectors (eg. ".pull-right > .dropdown"), the drilling down should only
be done if the selector does not already fully match the specified
selector. I've added a unit test to illustrate the fix, the existing
unit tests all still pass. The comments on pull request #323 refer to
this issue.