add h.264 support, but disable it by default

There is some patent and license stuff, so be aware of that before redistributing cef with h.264 support

v2:
* add option to remove flash support
* updated ebuilds
* documentation for H264_SUPPORT

NOTES:
* this is a candidate for the stable branch
* a full rebuild of cef is needed
* to test this you can simply remove all flash binaries