Add unit test for decoder test_vector_test

Add unit test for decoder test_vector_test

Got 61 test vectors from vp8-test-vectors.git
(http://git.chromium.org/gitweb/?p=webm/vp8-test-vectors.git)

Added decoder test vectors downloading in unit tests. Uploaded
the test vectors and their md5 files to WebM website.
  $ gsutil cp *.* gs://downloads.webmproject.org/test_data/libvpx

Added their sha1sum to the test/test-data.sha1 file.

In unit tests, download the test vectors to LIBVPX_TEST_DATA_PATH.

Test_vector_test goes through the test vectors, decodes them, and
compute the md5 checksums. The checksums are compared with the
expected md5 checksums to tell if the decoder decodes correctly.

Change-Id: Ia1e84f9347ddf1d4a02e056c0fee7d28dccfae15