config: Start adding basic validators + tests for new config values.
config: Add ConfigValue base class and tests.
config: Add String config value and tests.
config: Improve validate error messages and fix handling of non-string choices.
config: Add Integer ConfigValue and tests.
config: Add Boolean ConfigValue and tests.
config: Add List ConfigValue and tests.
config: Add LogLevel ConfigValue and tests.
config: Switch to non context manager version of assertRaises
config: Add Hostname and Port ConfigValues and tests.
config: Add config schema and tests.

Config schemas are used to group config values and check that each of them is
deserialized corretly, that none are missing and that there are no unkown keys
present.
config: Add ExtensionConfigSchema.
config: Add LogLevelConfigSchema.
config: Add mopidy.config to hold config schemas and eventually settings access and loading.
config: Address review comments.
Merge branch 'develop' into feature/config
config: re-add ConfigError.__iter__
main: Add ConfigError test.
Merge pull request #379 from adamcik/feature/config

Add mopidy.utils.config with new ConfigValue and ConfigSchema classes.