QSci has a bug where it will not display call tips in the GUI the first
time you enter a bracket for a function. You need to either enter an
argument and a comma, or delete the bracket and put it back in again.
Since jedi's completions are updated on every key movement the call tips
will never show unless they are persistent.

Re-factor 'get_function_spec' and 'generate_call_tips' out of the
CodeCompleter class and into standalone functions.

jedi crashes when launched using PyCharm's debugger on Python 2. Having
jedi as optional means we can still use the debugger whilst we're not
using Python 3.

Only add functions/builtins from simple api if they are title case and
don not begin with a '_'.

Add jedi's static code analysis completions to the CodeCompleter class.

Only generate the call tips if the simple api import is present in the
script.

Add CodeCompleter tests to CMakeLists

New CodeCompleter class contains the logic for populating QScintilla's
completion API. It's CodeCompleter.update_completion_api is called
whenever a script is successfully run.