Adds @placebo_session decorator

[Miserlou]

This adds the @placebo_session decorator mentioned in #40. Docs and test included.

This work is largely just done by @mathom - I'm just moving it upstream.

[garnaat]

Thanks!

[vascop]

Will this be pushed to pypi soon?

[garnaat]

Just pushed a 0.8.0

[vascop]

Thanks. I tried it and PLACEBO_DIR is going into __file__ which is wherever placebo is installed, in my case, my virtual environment. I also think that it should be configurable in some way. But definitely shouldn't be going into the virtual environment as a default.

[garnaat]

It seems like PLACEBO_DIR should be an environment variable?