compile with -O3
Merge branch 'master' into 2.x
add "cross platform" to feature list
update libav to 1c01b0253eb
build system: bundle SDL2 but try to use system SDL2 if possible
update README
when doing bundled SDL2, use the cmake build
enable SDL_File because osx needs it
Merge branch 'master' into 2.x

Conflicts:
	Makefile
try to build against system libebur128 and fall back on bundled. closes #38
Merge branch 'master' into 2.x

Conflicts:
	CMakeLists.txt