Make config keys (sections and names) case insensitive

This is done to match the behaviour of git itself.

The main change here is to add the lower_key function
and the CaseInsensitiveDict class.

lower_key coerces keys that are used in DictConfig to
make sure all keys consist only of lower-cased strings.

CaseInsensitiveDict has altered getitem and
setitem methods to call lower_key on the key for
both get and set operations.

CaseInsensitiveDict.make takes care of the creation
logic found in ConfigDict.init as well as converting
any passed-in dicts to CaseInsensitiveDict.

CaseInsensitiveDict.get and CaseInsensitiveDict.setdefault
allow the default value returned to be a CaseInsensitiveDict
without repeating it everywhere.

ConfigDict.check_section_and_name just moves some logic
that was previously repeated.

Fixes #599