Adding support for tag names in XML to have dots(.) inside as per XML spec

This pull request implements the feature described in [#355](https://github.com/haml/haml/issues/355). According to the XML spec xml tag names can contain dots(.). To change consisted in modifying the regular expression used to parse a tag description to be slightly different when parsing an xml. 
This pull request contains:
- Test for xml containing tag names with dots
- Updated the just_stuff template test to take into consideration the feature request
- Added logic to determine if the parser is parsing an xml and return different regexp to parse the tag description
- Extracted the logic above into a private method