This diff does a couple of things; all new features come from requests made by our animators (@Hjalti Hjálmarsson (hjalti) mostly).

Note that here this is all shown as one change, but there are multiple separate commits for cleanup and each feature.

• New feature: Shift+Alt+W shows a popup menu with the currently available selection sets. This allows our animators to quickly use the selection sets from within the 3D View.
• New feature: Copy & Paste of selection sets. The copied Selection Set is stored as JSON document on the clipboard, like {"name": "SelectionSet", "bones": ["Bone", "Bone.001"]}. Using JSON allows the animator to copy/paste selection sets through text-based communication channels (IRC, email, bug reports, etc.). When pasting, a new Selection Set is always made, regardless of whether one with the same name already exists.
• Cleanup: simplified some poll() methods. Inverting the condition allows for simpler code and non-conditional core functionality.
• Cleanup: using cls instead of self in poll() methods, as they are class methods and don't receive a 'self'.
• Cleanup: PEP-8 formatting
• Bumped version from 2.0.1 → 2.1.0