Placebo

This is the first placebo release in almost three years. The main changes are:

- Fixed a bug (see 79 and 70) with binary response bodies from services.
- Dropped support for Python 2.x

Allow more configuration of placebo decorator ( 44 )

Added a decorate. Also made placebo compatible with Python 2.6.

Fixed a bug ( 33 ) involving clients created before the `playback` or `record` method of the pill was called.

Added a `session` property to Pill object to retrieve the session the pill is associated with.

Added simple support for `StreamingBody` elements in responses. See 12