Several fixes to XML and Niflib to enable all my files from Freedom Force (except with NiClodData), Oblivion, CivIV, Morrowind, Zoo Tycoon 2, and DAoC to load in Niflib and NifSkope.
Includes the two strange Oblivion files with version 3.3.0.13 and version 10.0.1.2.

CheckNifHeader -> GetNifVersion
GetVersion -> ParseVersionString
IsVersionSupported -> IsSupportedVersion
Added new function, FormatVersionString, to format integer version numbers as a #.#.#.# string.
GetNifVersion so it no longer returns VER_UNSUPPORTED.  It now always returns the actual version value, if possible, which can be passed to IsSupportedVersion if desired.
GetNifVersion now supports version numbers lower than 4.
Added versions 4.0.0.0 and 10.1.0.106 to version constants and IsSupportedVersion function.
Updated some documentation.

Moved version constants to a separate file to clear up problems with no longer including NIF_IO.h in niflib.h.