Build into include/GRMustache in the built products directory.

Allows a framework-style #import <GRMustache/GRMustache.h>, and no need to set a custom include search path.
GRMustacheKeyValidation WIP
GRMustacheKeyValidation WIP
Drop support for GRMustacheContext managed properties
Drop support for GRMustacheContext subclasses
Drop deprecated -[GRMustacheContext valueForMustacheExpression:error:]
GRMustacheKeyValidation WIP
GRMustacheKeyValidation WIP
New method -[id<GRMustacheTagDelegate> mustacheTag:willRender:]
Remove method -[id<GRMustacheTagDelegate> mustacheTag:willRender:], test that tagDelegate can alter the rendering of objects (not only the rendered objects), and test that the tag delegates of the standard library use this feature.
Faster string escaping
Optimization of key validation & GRMustacheTagDelegate conformance test
Caches
Introduce GRMustacheFastBuffer, an efficient string buffer
GRMustacheFastBuffer -> GRMustacheBuffer
GRMustacheStringSet WIP
GRMustacheStringSet WIP
Remove GRMustacheStringSet
GRMustacheBuffer WIP
less TODO