Add default value

Change documentation
Add default value for direct mapping
Check if model is a placeholder before we override existing CoreData properties
Throw an error if JSON mappings to match property keys
Revert "Check if model is a placeholder before we override existing CoreData properties"

This reverts commit 0c52681f3648d93ccb8419f70ba7427dfd7c6502.
Merge branch 'master' of github.com:MantleFramework/Mantle into hack/check_if_placeholder_before_overwriting_coredata
MTLManagedObjectSerializing now allows merging values before overwriting
them.
Added convenience functions for arrays of JSON/Models

You can now use [MTLJSONAdapter modelsOfClass: fromJSONArray: to convert an array of JSON dictionaries to models and then [MTLJSONAdapter JSONArrayForModels] to get a JSON array of dictionaries from a array of models.  Closes #176.
A bit of documentation clean up
Code cleanup in response to PR comments
Fixed a few more Tab/whitespace issues
Clean up formatting, variable names.
Remove .idea files from project
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.