Fix AttributeDescriptor.__set__.

    * AttributeDescriptor.__set__ will never be called in
      its class context (instance=None), then remove the code.
    * However, it IS called under IronPython
      (http://ironpython.codeplex.com/workitem/29192).
      Add ipy detection & test compat adaptation.
Allow subclassing Froshki subclasses.
Refactor.

    * Move upper Attribute & AttributeDescriptor classes.
    * Refactor Froshki.__new__.
Add git-ignore.
Increment version.

    * Mainly for Froshki subclassing extension.