Add demo project
Added documentation to main file
New OGTypes class for converting tags to strings
Added prefixes to string methods and documented them
Refactored OGText to make it safer

Improved general API (incompatible with old one) and used new Objective
Gumbo types.
Correction to enum used
Documentation of OGNode
Refactored OGNode implementation to match interface
Made OGDocument immutable
OGNode is now immutable
OGText now uses parent initialiser
Filtering and indentation methods changed slightly
Document now initialised using initWithParent
Immutable OGElement

OGElement’s public API is now totally immutable. However, there is now
a mutable class extension which declares the setter methods. This can
be used within Objective Gumbo only to set the properties of an element.
Removal of old code from ObjectiveGumbo parser class
New York Times style code
Type matching and removed redundant code
Rename group in Xcode
Replaced initWithParent with mutability support

Mutability support is declared within a private header file
Changes to API usage in Objective Gumbo