Add buildbot badges from nightlies to readme

Adds buildbot badges from latest master nightly to readme, and linkifies first two occurances of libavg in text

Badges look just like this. (There are prettier ones, but those are buildbot defaults…)
Windows: [![Build Status](http://libavg.de:8010/png?builder=i386-win-full)](http://libavg.de:8010/builders/i386-win-full)
Linux: [![Build Status](http://libavg.de:8010/png?builder=x64-lnx-full)](http://libavg.de:8010/builders/x64-lnx-full)
OSX: [![Build Status](http://libavg.de:8010/png?builder=x64-osx-full)](http://libavg.de:8010/builders/x64-osx-full)