meta.h: inherit from TrueType/FalseType where applicable

To reduce repetition and to provide a proper type hierarchy,
let type traits inherit from TrueType and FalseType.
Add a wrapper for constant string references
Add documentation to new string API
Doxyfile: expand internal RAPIDJSON_DISABLEIF_RETURN macro
update string API tests
GenericStringRef: disallow assignment, const string pointer
GenericValue: fixup bool constructor

With the new string handling API, the constructor taking a `bool`
parameter matches in some unwanted cases, as pointers can be casted
to `bool` implicitly.

Add a SFINAE helper to this constructor to avoid matching arbitrary
pointers.  To avoid confusion for the user, this mechanism is hidden
from the Doxygen documentation.
adopt AddMember tests from #47 (726f986,b7bf73c)
GenericValue: add Move()
tutorial.md: document GenericValue::Move()
tutorial.md: update for new string API
Merge pull request #57 from pah/cleanup/string-handling

Improved handling of (constant) strings