scanner/config: Convert to new config system (fixes #405)
Merge pull request #412 from adamcik/feature/scanner-to-new-config

scanner/config: Convert to new config system (fixes #405)
utils: Remove unused importing utils
exc: Remove unused SettingsError
exc: Remove OptionalDependencyError

It is no longer needed as the extension loading mechanisms verifies the
environment and presence of dependencies before it tries to import modules that
would cause ImportErrors if optional dependencies are missing.
docs: Update default.conf path
Make setup.py Python 3 compatible
Remove tox.ini

Testing across Python 2.6 and 2.7 with tox is no longer working on systems
which doesn't ship both Python versions by default. I can install Python 2.6 on
a newer Ubuntu system using the deadsnakes PPA, but dependencies installed from
APT, like gobject and gstreamer are still only installed for Python 2.7, thus
making tox useless.
docs: Document API of Extension class
docs: Update changelog
docs: Fix typo
docs: Remove b'' usage in setup.py example
docs: Document config API
docs: More reasons for running 'python setup.py develop'
docs: Don't number lists explicitly in rST
docs: Add note about using virtualenv
docs: Fix link from Ext API to Config API
docs: Include Backend class in API docs
docs: Fix class links in Ext API docs