adds and option on startup to move to a different subfolder

[gureckis]

currently the Heroku set up requires all the files to be in the top level folder. however some times it would be convenient to have multiple experiments in a single github folder and to swap between them. this adds an option (None by default) to let you configure this. the tip to use it would be, on Heroku, to change the heroku_app.py file so that you pass a directory to exp.launch(). For example, if your experiment was in ./exp1 then exp.launch("./exp1"). the config.txt should be placed inside exp1 folder in this instance as well as static, templates and the custom.py file.

[coveralls]

Coverage decreased (-0.04%) to 59.891% when pulling d781d4f on run_from_subfolder_heroku into d81d353 on master.

[gureckis]

I just verified this minor change works from this branch to run from a subfolder... so let 'er rip (and be ignored by most people)