Add initial support for gstreamer 1.x.
* Modified configure.ac to check first for the latest gstreamer version available (1.x), if it is not available, it will fallback to the old version (0.10.x).
* If there is not a gstreamer version available it will fail.
* Set valaflags, cflags and ldadd according to the gstreamer version available.
* Link against gstreamer-base and gstreamer-video for version 1.x.