Gst::CapsFeatures: wrapped GstCapsFeatures boxed structure

    * .gitignore: added capsfeatures generated files.
    * gstreamer/gstreamermm.h: added capsfeatures header to main header
      file.
    * gstreamer/src/capsfeatures.{ccg|hg}: implementation of
      GstCapsFeatures wrapper.
    * gstreamer/src/filelist.hg: added new wrapper to a build file list.
    * tests/Makefile.am: added capsfeatures class test file to a testfile
      list.
    * tests/test-capsfeatures.cc: added a few simple capsfeatures tests.
    * tools/m4/convert_gst.m4: added conversions between GstCapsFeatures
      and its wrapper.
Gst::Caps: added methods interrelated with Gst::CapsFeatures class

    * gstreamer/src/caps.{ccg|hg}: wrapped following methods:
      gst_caps_append_structure_full, gst_caps_merge_structure_full,
      gst_caps_get_features, gst_caps_set_features.