Use xmlBuf* accessors to get values from libxml2 buffer.

With version >= 2.9.0 of libxml2 xmlBuf struct elements can no longer
be accessed directly. Instead, need to use dedicated functions.

In this change:
 * modify modxslt.h to include libxml/xmlversion.h
 * add code to use new accessors if libxml2 >= 2.9.0.

This change closes #5.