Added new virtual_table class and template support.

Dimtable now attempts to sort items in each dimension numerically, then
lexically.
Updated html.py to python 3.
Added new KeyedTable class, designed to work with a data-set that takes
keyed input.
Moved dimtable.css to static folder, so that it can be used with
staticfiles.
Cleaned up imports.
Added new virtual_table class, which does not generate html on its own,
and is intended to be used in conjunction with the new multi_table
template tag. Eventually, this will completely replace the old dim_table
class.
Added default templates.
Merge branch 'master' of https://github.com/rmehlinger-spirent/dimtable