Replace libxml++ w/ libxml2

[rakhimov]

XML parser facilities are only needed to process MEF and config files. SCRAM does not need all the bells & whistles provided by libxml++. It should be possible to replace libxml++ with a custom minimal adapter around libxml2, providing only the needed functionality. This would free SCRAM from libxml++ and its dependencies (glib, glibmm, libsigc++).

Moreover, the performance should improve slightly if the custom wrapper does not manage wrapper node databases and XML modification as libxml++ does.

The codebase needs to be updated for the new libxml++3 API anyway, so this is a good excuse for creating the custom adaptor and dropping libxml++.

Required features:

• [x] DOM parser & read-only document (from file & stream)
• [x] XInclude processing (automatic w/ libxml2)
• [x] RelaxNG validator
• [x] Element nodes only (w/ attribute & text values)

[rakhimov]

libxml++ is fully replaced by libxml2 custom wrapper, but error message construction is not handled. This is delegated to #219 .