move cache responsibilities to config_store.rb
Merge pull request #139 from bolandrm/add_config_cache_class

move cache responsibilities to config_store.rb
More reliable `for` keyword check
Handle defs with names coinciding with keywords
Add an extra example to AvoidFor's spec
Improve a bit ArrayLiteral & HashLiteral cops
Add possibility to use inherit_from in .rubocop.yml.

This adds functionality for inherit_from and uses it in the new
config/*.yml files, and also from our own .rubocop.yml.

Default when no .rubocop.yml is found is to use config/default.yml,
and all configuration files inherit implicitly from config/default.yml.

Default values are not handled in code anymore, since full
configuration is always present.