`py-compute-indentation' in Python shell fixed

`py-count-lines' must count from last prompt
py--send-string-...

Filter output only if non-empty string
py-count-lines again
TAB completion not working, lp:1325474, fixed

New command `py-shell-complete-or-indent':

Complete or indent depending on the context.

If cursor is at current-indentation and further indent
seems reasonable, indent. Otherwise try to complete.