Use symbols rather than modules in ispoisoned

[timholy]

This allows users to poison packages before they are loaded, forcing manual regeneration of the cached files. This enables the syntax in the README in #3.

[vtjnash]

this cache needs to be able to track the fullname of a module. it is also preferably able to "unpoison" the module after it has been defined once. I agree that this is needed – just not certain of the details yet. I think if you could switch it to using fullname, I'll go ahead and merge, since this entire enterprise is really a WIP anyways

[timholy]

OK, this uses the fullname.

As an alternative to unpoisoning, perhaps the better solution is to maintain a separate cache for "poison-once" packages. Speed.regenerate!(:Images)? As it is, at least now users can start julia, say Speed.poison(:Images); using Images, quit julia & restart, and then they have what they want.