Renamed the following functions:

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.