more comprehensive and specific gitignore

mostly ignoring autoconf and automake stuff

I understand if you don't want this change, it's a matter of taste, really.

Notice that I don't just ignore `*.o` - I try to be as specific as possible to avoid ignoring "unexpected" files, which I like to be shown in `git status`. I use leading `/` where appropriate to anchor to the root of the repository, so the same file or folder name elsewhere won't be ignored. I intentionally don't ignore OS-specific or editor-specific files.

before:
```
[pierce@plo-pro janus-gateway]$ git st
## master...ploxiln/master
?? .deps/
?? Makefile
?? Makefile.in
?? aclocal.m4
?? autom4te.cache/
?? compile
?? conf/janus.cfg.sample
?? conf/janus.plugin.recordplay.cfg.sample
?? conf/janus.plugin.streaming.cfg.sample
?? config.guess
?? config.log
?? config.status
?? config.sub
?? configure
?? depcomp
?? docs/Makefile
?? docs/Makefile.in
?? html/Makefile
?? html/Makefile.in
?? install-sh
?? janus-apierror.o
?? janus-cmdline.o
?? janus-config.o
?? janus-dtls.o
?? janus-ice.o
?? janus-janus.o
?? janus-pp-rec
?? janus-record.o
?? janus-rtcp.o
?? janus-sctp.o
?? janus-sdp.o
?? janus-utils.o
?? libtool
?? ltmain.sh
?? m4/
?? missing
?? plugins/.deps/
?? plugins/.dirstamp
?? plugins/.libs/
?? plugins/janus-plugin.o
?? plugins/libjanus_audiobridge.la
?? plugins/libjanus_echotest.la
?? plugins/libjanus_recordplay.la
?? plugins/libjanus_streaming.la
?? plugins/libjanus_videocall.la
?? plugins/libjanus_videoroom.la
?? plugins/libjanus_voicemail.la
?? plugins/plugins_libjanus_audiobridge_la-janus_audiobridge.lo
?? plugins/plugins_libjanus_audiobridge_la-janus_audiobridge.o
?? plugins/plugins_libjanus_echotest_la-janus_echotest.lo
?? plugins/plugins_libjanus_echotest_la-janus_echotest.o
?? plugins/plugins_libjanus_recordplay_la-janus_recordplay.lo
?? plugins/plugins_libjanus_recordplay_la-janus_recordplay.o
?? plugins/plugins_libjanus_streaming_la-janus_streaming.lo
?? plugins/plugins_libjanus_streaming_la-janus_streaming.o
?? plugins/plugins_libjanus_videocall_la-janus_videocall.lo
?? plugins/plugins_libjanus_videocall_la-janus_videocall.o
?? plugins/plugins_libjanus_videoroom_la-janus_videoroom.lo
?? plugins/plugins_libjanus_videoroom_la-janus_videoroom.o
?? plugins/plugins_libjanus_voicemail_la-janus_voicemail.lo
?? plugins/plugins_libjanus_voicemail_la-janus_voicemail.o
?? postprocessing/.deps/
?? postprocessing/.dirstamp
?? postprocessing/janus_pp_rec-janus-pp-rec.o
?? postprocessing/janus_pp_rec-pp-opus.o
?? postprocessing/janus_pp_rec-pp-webm.o
```

after:
```
[pierce@plo-pro janus-gateway]$ git st
## more_gitignore...ploxiln/more_gitignore
?? conf/janus.cfg.sample
?? conf/janus.plugin.recordplay.cfg.sample
?? conf/janus.plugin.streaming.cfg.sample
```

(I'm content to let those configs be for now)