Moved the API patch from NextGenv2. An example was included.
To try it, for example, run the following command:
$ examples/vpx_cx_set_ref vp9 352 288 in.yuv out.ivf 4 30

Change-Id: I4cf8f23b86d7ebd85ffd2630dcfbd799c0b88101

Specifying the --prefix command line arg executes all test programs within the
context of the prefix string, which is assigned to VPX_TEST_PREFIX.
All test functions updated to include VPX_TEST_PREFIX in their eval command.

Change-Id: I2e215cc8f216048edf3269db02a6b5660fe32318

Add elog() to tools_common, and checks for the executable in each test.

Change-Id: I3c6334eca62c0b56040d91145abbe1bf5f9c763c

Change-Id: I68850699ecaebbbb7962cbc1c9bc85e92a0ea5ca