docs: Add Debian package updating to the release procedure
docs: Simplify Extension class by removing @classmethod
docs: Refine extensiondev docs
docs: Sort imports
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
config: Start adding basic validators + tests for new config values.
config: Add ConfigValue base class and tests.
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
config: Add String config value and tests.