Enable semicolon for MixinCall and MixinDefinition arguments

This includes Pull Request 323 and I believe addresses Issue 319, Issue 320 and Issue 321 (which are all basically the same). The comments in Pull Request 323 are all also addressed by other recent fixes. The Bootstrap 3 stylesheets now compile without any error! (I haven't yet compared the output from dotLess for Bootstrap with that from less.js but it's looking pretty good in its current state).