[WIP] Update philosophy section

As @kennethreitz and I discussed at PyCon, this is a first pass at a rewrite of part of the Philosophy section of the docs, attempting to explain Requests' slightly unusual management style. I'd like some feedback.

Some ideas:

- should @shazow be mentioned? I felt like he should but I wasn't sure how best to explain the relationship.
- is there anything major that I've missed?