remove os.chdir;  fix logging output; closes #601

Each builder script now does its own os.chdir at the start of the script when its run. I think this is the right place for this to happen (rather than inside Builder), and it has fixed some errors during testing (os.getcwd: no such directory). I think this should be fully backwards compatible, just more robust. I cleaned up some minor things, and added an explicit test of info.py, no actual tests yet but a framework. (Sorry for the messy commit message--this was my first use of --squash). 

Also fixed the recent logging clutter by passing along the requested log value in basevisual.BaseVisualStim._set