ext: Add Extension base class
tests: Test existing exception classes
ext: Add ExtensionError exception class
ext: Support multiple frontends/backends in an extension
Merge pull request #372 from jodal/feature/add-extension-superclass

Add extension superclass and exception
Fix error in MANIFEST.in
Ignore *.egg-info files generated by setuptools
Switch from distutils to setuptools
docs: setuptools always regenerates MANIFEST
Let setuptools generate executables
docs: Remove 'platforms' from extension's setup.py example
docs: If using pip Pykka will be installed automatically
docs: Fix typo
docs: Include https in links where appropriate
Merge pull request #373 from jodal/feature/use-setuptools

Switch from distutils to setuptools
Update Pykka version check