Organized and documented the code
Add README; update license
Changes to Dump and pretty printing

Now we show the Sequent Core code pre-optimization (much more readable). Also,
use an underscore for the "box" rather than [] because that's an empty list.
Added content to the README file.
Flesh out README; tweak dump syntax

Got rid of the _ in the syntax for frames, so `f 4` translates to `<f | $ 4>`.
Also, drop the angle brackets and the pipe when the continuation is empty.

In the README, besides revising the examples to match the syntax changes,
mostly I just added a bit about how the Sequent Core types are really just the
Core type split into three.
Prepare sequent-core.cabal for Hackage upload
Cosmetic tweaks
Merge from lukemaurer

Conflicts:
	src/SequentPlugin.hs