Merge pull request #73 from goinnn/master

Two little errors after the merge
Add tox.ini
Fix pylint violations
Fix link column tests.
Fix testing infrastructure.
Add more test environments
Instantiate ``BoundColumn``s only once when table is first instantiated instead of every time a row is being read.
Make table.sequence default to ...
Store fully expanded sequence in _sequence
Moved table._sequence to table._sequence_expanded for backwards compatibility.
Revert "Moved table._sequence to table._sequence_expanded for backwards compatibility."

This reverts commit 1debcfe942a19dffbe8253162ee8b1696e642a81.
Table.sequence should respect ._meta.sequence.
A few minor tweaks and fixes.
Move ``funcs`` and ``specs`` in columns.py to module level.
Move ``bound_column.render_args`` to ``bound_column._render_args``.
Turn warnings into errors when running tests, use latest Attest
Switch 'Helvetica' and 'Verdana' in CSS for table headings.
Column.order_by now defaults to Column.accessor instead of Column.name, resolves issue #74
Change tox.ini to use tarballs from github for latest releases, rather than checking out the entire repo
Setting Table.Meta.fields now affects default sequence of columns. Resolves #76
Preserve whitespace in headings and body of table HTML, resolves #80