Added basic GeologicalStrata object
Code reformatting and documentation improvements.
Added Strata object to FossilOccurence, speedups

Because FossilOccurence results from PBDB contain information about taxa and
strata, I've added functionality to pre-populate the related object caches
with information to save them a trip to the PBDB.
Merge pull request #2 from myfossil/enh/geochronology

Adds geological strata and lithostratigraphy to FossilOccurence objects