implement SessionViewer

tenk/sessionviewer.py: Class which provides session data viewing
functionality for most recent sessions or a given date
tenk/sessions.py: Added a class method, deserialize, which returns a
Session object given a skill name and session tree
some bug fixes in sessionviewer

tenk/sessionviewer.py: handle cases where session data does not exist or
the skill does not exist.
update CLI with session viewing options

tkcli/main.py: add view option to allow user to easily view session data
README.md: add usage examples for new view option
Merge branch 'sessionviewer' into dev