Add layer group object #60

Needs to be in new file, the struct should implement Stringer, as the other objects do.

Add a test tmx file to testdata and ensure it is read. Also need additional test file with any specific LayerGroup exported functions tested (String() excluded).

Struct should have the following methods:

• String() string - returns human readable representation of the object
• setParent(*Map) - sets the parent map, and sets parent map of all children, where appropriate
• GetXLayerByName(string) X - same as those on Map, need to be able to get all three layer types