Fixed compilation issues with MSVC 6: replace usage of ostringstream with valueToString to support 64 bits integer and high precision floating point conversion to string. Replace usage of ULL and LL literal with UInt64(expr) and Int64(expr). Introduced helper function uint64ToDouble() to work-around missing conversion. Unit tests do not pass yet.

git-svn-id: http://jsoncpp.svn.sourceforge.net/svnroot/jsoncpp/trunk@234 1f120ed1-78a5-a849-adca-83f0a9e25bb6