Master names: if parts of name not in predefined MASTER_NAME_*, mishandling might occur

In the test font `BraceTestFont.glyphs` included in https://github.com/googlei18n/glyphsLib/pull/478, I noticed that when I named the "Light Condensed" master "Thin Condensed", the final style name was "Thin Condensed" whereas "Bold Condensed" was "Condensed Bold" (the test font intentionally places the width axis before the weight axis). The "Light Condensed" master's `_name` attribute was set to the name, but it wasn't set in the "Bold Condensed" master. I suppose this is because "Thin" does not appear in the list at https://github.com/googlei18n/glyphsLib/blob/23f6290755adef5107f85921462718de82f2578f/Lib/glyphsLib/classes.py#L1326.

Explicit master naming was introduced in 2.5 if I remember correctly... Maybe the code needs to be changed to assume the new naming way and prefer the master name for the style name?