Make tests runnable from lifelines.tests

This commit allows the following syntax:

    python -m lifelines.tests

Which can come in handy for refactoring tests into several
test-suites.

Signed-off-by: Jonas Kalderstam <jonas@kalderstam.se>
Move all datasets into datasets-subpackage

Datasets were not installed into the correct package directory when
using pip. This commit puts all dataset-related functionality in
package 'lifelines.datasets'.

Due to location of datasets not being known when package is installed,
convenience functions have been added to load the datasets using
pkg_resource.

Previous functionality in 'datasets.py' has been moved into
'lifelines.datasets.__init__.py' to remain backwards-compatible.

Fixes #98

Signed-off-by: Jonas Kalderstam <jonas@kalderstam.se>
PEP8 fixes

Signed-off-by: Jonas Kalderstam <jonas@kalderstam.se>
Merge pull request #99 from CamDavidsonPilon/test_improvements

Fix dataset inclusion