Add default value

Change documentation
Add default value for direct mapping
Merge branch 'master' of github.com:DAlOG/Mantle into enum-mapping-default-value

Conflicts:
	Mantle/NSValueTransformer+MTLPredefinedTransformerAdditions.m
WIP: Spec for default value when transforming enum values.
WIP: Fix incorrect merge conflict resolution.
Update documentation.
Remove whitespace.
Update spec and method for reverse default value.
Assert that default values are provided in pairs.
Remove whitespace.
Remove whitespace.
Fix outdated method in documentation.
Remove unwanted assertion.
Updated documentation.
Merge pull request #304 from paulyoung/enum-mapping-default-value

Enum mapping default value