Gst::Context: wrapped GstContext struct

    * .gitignore: added generated files to ignored list.
    * MSVC_2010/gstreamermm/gstreamermm.vcxproj:
    * MSVC_2010/gstreamermm/gstreamermm.vcxproj.filters: added context
      files to a msvc project.
    * gstreamer/gstreamermm.h: added context header to a common include
      file.
    * gstreamer/src/context.ccg:
    * gstreamer/src/context.hg: added wrappers for GstContext structure.
    * gstreamer/src/filelist.am: added context wrapper to build file list.
    * tools/m4/convert_gst.m4: a few conversions between GstContext and
      Gst::Context added.
Gst::Element: wrapped a few methods

    * gstreamer/src/element.hg: wrapped set_context method and set_context
      virtual function.