port to aspen

[chadwhitacre]

The plan on AspenWeb/salon#2 is to split out an aspen-core package that contains simplates and dispatch. Once that's done (see #527) we should port aspen.py to use that.

[Changaco]

@whit537 Are you going to take this on, or should I? You're the one who knows the code best, so you might finish faster, but I can get it done too.

[chadwhitacre]

@Changaco Go for it, I may end up adding some commits during review.

[Changaco]

Tests are passing on my local use-aspen branch. \o/

[Changaco]

And tests are also passing on my local pando liberapay branch (with a few more tweaks). \o/