GenericValue: fixup construction from NumberType

Constructing an empty GenericValue with a specific Type has failed
for any kNumberType (Int, Int64, Double...) due to incomplete definition
of the corresponding default flag value.

This patch adds a new constant kNumberAnyFlag to the flags enumeration
in GenericValue to cover this case.

This fixes http://code.google.com/p/rapidjson/issues/detail?id=57