[Session] make tmpo path configurable at Session init time
[series] select max(rid) by default
[Session] do not hardcode path separator
[Session] add a dataframe method
[df] cf Roel's take-home message: _always_ pass a tz
[df] allow Timestamp as type for head/tail
[series] default index to datetime

This breaks compatibility with previous commits for which epochs were used
as the series index. Bumping tmpo to v0.2.0.
[ipynb] add demo ipython notebook
[py] make tmpo compatible with Python 3.x