#5 Fix compile failure: missing <tr1/unordered_map> for modern compilers

[kbinani]

scons install failed with messages:

/usr/local/include/cppdom-1.2.0/cppdom/cppdom.h:82:18: fatal error: 'tr1/unordered_map' file not found
#        include <tr1/unordered_map>
                 ^
1 error generated.
scons: *** [build.darwin/type-optimized--arch-x64/cppdom/cppdom.o] Error 1
scons: building terminated because of errors.

on MacOSX10.9.4, Xcode 5.1.1.

The patch fixes this issue by using std::unordered_map if std::tr1::unordered_map is not available.