Fix scripts/gen_bridge_descriptors to create two cached-extrainfo files.

We usually have 'cached-extrainfo' and 'cached-extrainfo.new', not
'extra-infos'.
Update README with new commands.

 * FIX also the markdown formatting of indented blocks in some places.
Update bridgedb.conf.

 * REMOVE RUN_IN_DIR
 * CHANGE EXTRA_INFO_FILES to a list with the correct filenames.
 * CHANGE HTTPS options to bind to 127.0.0.1:6789 by default (for testing).
 * ADD missing HTTP_N_BRIDGES_PER_ANSWER option.
 * CHANGE to enabling bridge fingerprints by default.
 * CHANGE to GPG signing emails by default.
Update .travis.tml to use new command syntax.
Add expired GPG secret subkeys, marked "TESTING (INSECURE!)".

 * ADD GPG keys for testing purposes. They are expired, passwordless, marked
   with the UID "TESTING (INSECURE!) <bridges@bridges.torproject.org>". The
   (INSECURE!) part tells GnuPG to not waste real entropy when using this key.
Add BridgeDB's real GPG public keys.

 * ADD 16384-bit public half of offline certification-only key.
 * ADD 4096-bit public halves of online signing+encryption keys.
 * ADD gnupghome/gpg.conf file with settings (both for the fake testing key
   and the real keys).
 * ADD gnupghome/* to the .gitignore.
Add placeholder file for lib/bridgedb/test package directory.
Remove old bridgedb.Opt module.