make install: Build stbt-gst-plugins.so if necessary

Fixes https://github.com/drothlis/stb-tester/issues/230
Makefile: `enable_stbt_camera=no` does what you'd expect

Previously only an empty value for `enable_stbt_camera` would disable
it.
Makefile: Cleanup: `stbt-camera` isn't a build artifact
Makefile: Cleanup spacing of ":" in rules

For consistency, and so that I can find the "install" target (as opposed
to other targets like "install-core" or "install" commands in recipes)
by searching for "install:".
Remove extra/fedora/Vagrant*

(These were to provide a host linux system for running docker in, to
build stb-tester's Fedora packages inside a docker container.)

With docker 1.3, boot2docker for OS X supports shared directories on the
host, so vagrant for a host linux system isn't necessary any more.
Anyway I have a Linux host system now.
tests/run-tests.sh: Cleanup: Rename internal variable

"test_installation" is ambiguous: Is it a verb (test the installation)
or a noun (the test installation)? If it's a verb, does it mean to test
the installation process or to test the version that's already
installed? (It's the latter.)
extra/fedora: Use the (semi-) official Fedora docker image
make check: Run stbt-camera tests sequentially, and not at all if stbt-camera disabled

Fixes https://github.com/drothlis/stb-tester/issues/233

This implementation seems less error-prone than remembering to add
"skip" at the beginning of each new test case in test-camera.sh.

This also runs the stbt-camera tests sequentially. Running them in
parallel was *killing* the responsiveness of other apps on my computer,
with various python processes at 160%+ CPU usage.

Running sequentially might also make the tests more reliable (though
some of them are still consistently failing on my computer).
Makefile: Require enabling "stbt camera" support explicitly

There are a number of outstanding issues with the "stbt camera"
self-tests, so to avoid inconveniencing users who want to run "make
check" but aren't interested in the camera support, this has to be
enabled explicitly with "make enable_stbt_camera=yes".
Merge pull request #235 from drothlis/stbt-camera-makefile-fixes

Miscellaneous build fixes
release notes: Mention new `stbt camera` command, tidy up wording elsewhere
stbt camera README: Document `stbt --with-experimental` flag
stbt camera README: Document voaacenc dependency on Fedora
release notes: List all changes since 0.20 that were missing