Change cpp from innermost to outermost

Closes #116.
Use nrepl load-file for :[range]Eval

Closes #83.
Accidental mutation
Slightly more elaborate hack to detect final nREPL response
Different gross hack for extracting backtrace

Now instead of wrapping the code up in a try/catch, we just use a series
of evals to grab *e and then restore *1/*2/*3.
Encapsulate user ns
Allow overriding buffer namespace
Fix :Connect with no arguments

Closes #120.
Fix :Connect with argument

References #120.
Fix debugging statement
Python all the way
So many missing aborts
Allow debugging connection failures
Generalize interface between Vim and Python
Restore accidentally deleted polling
Default to sending session id
Send and filter by id
Fix warning misfire about broken sessions
Abort python processes on Vim exit

You'd think a scripting language could handle portably checking if a pid
is still running but no.
Enable easier testing of Python exec