Depend on simplejson for Python < 2.7

This adds a dependency on simplejson for Python < 2.7 and moves the
stanza to import the appropriate json module into pyleus.compat.

Closes #46
In this way we are preventing the use of simplejson for python 2.7 and 3.x, right? I know that a try/catch import here  misses the point of this change (even though it would still simplify all other modules), but I feel we are losing something that might be useful (e.g. from my measurements, simplejson seemed to achieve lower latencies than json while processing tuples).

Nevertheless, I don't want to block the pull request for such an issue, since we ship messagepack as default encoding format and simplejson has problems too (https://code.google.com/p/simplejson/issues/detail?id=40). I just wanted to point it out.