Consolidated beagle viewers.
TfModel instances now remember their document names
random_corpus labels tokens according to `tok_name`
TfViewer updated with new sim functions and collection frequency methods.
Updated TfIdfViewer with new sim fns.
Added `tok_name` to BaseModel and I/O methods.
Updated LsaViewer to use new sim functions.
Merge branch 'develop' of https://github.com/inpho/vsm into develop