e-xml: Add C api

Makes it easier to use with C projects (especially those that don't allow C++ code)
Nice to see someone writing a test program for their change. I know Ben Chaco is working on a regression test infrastructure for epiphany-libs. Not sure if he has a preference for where tests should be put.

BTW, since you define e_xml_version, could you add a call to this program and then the test would be comprehensive.