Changes reqd for python3.3

Switched from Python2.7, virtualenv[wrapper],
to Python3.3 which comes with 'venv' built-in.
For some reason this required changing an import
in toml/__init__.py, not sure why.
Add groups.

Now parse '[group]' elements. Assignments that precede all groups
are put into same top-level dict as the groups. Assignments that
follow a group are put into a dict which is put into the top-level
dict under the group name.

System test now failing on a string value with embedded "\n".
Replace newline char in test data with backslash & 'n'.

From spec: 'Strings are single line values'. Hence newline char in string
(as was formerly in test data) is not legal. The intent is that a slash n
in a string is unescaped into a newline in the output data.
now lex date literals, but don't yet return correct value for them