Add VCF header representation, update info format

As discussed on Wednesday, this is a very basic representation of the header info in VCF. Nothing snazzy is going on here, this is just a data dump for v0.5.1 - in the next real version of the API we can do something way more intelligent (and work on all those other first class fields we should have :)

Also as previously discussed - this makes the info fields map to arrays of strings as a temporary hack fix for v0.5.1. (We'll still need to do a better job in the future, just patching for now!)