Drop Python 3.2 support, prep for D-Base32

[Jason Gerard DeRose]

So that we have a smaller target, and don't spend so much time maintaining back-ports, we're going to be dropping support for Precise and Quantal as we make the final steps toward Dmedia being production ready:

http://blog.novacut.com/2013/02/dropping-support-for-ubuntu-precise-and.html

One of the breaking changes will be switching from standard RFC-3548 Base32 encoding to our proposed D-Base32 encoding, the rationale for which is here:

http://docs.novacut.com/dbase32/design.html

We need to bring up everything for the V0 => V1 switch bit by bit, but not actually make the switch till everything is ready.

D-Base32 will only support Python 3.3 and newer, so as a step toward making the switch, we need to drop Python 3.2 support in FileStore.

Nicely, this means we can drop the back-ported posix_fadvise() extension we've been carrying.