Check whether output from obj.safely_stringify_for_pudb() is actually a string, so as to not be confused by Mock objects (closes #270 on github)

Fix keyword arg error

fix lint errors

Add ptipython

Don't show "Return: None" when "returning" from a module frame

That is, when the last line of a module file is executed (usually from
runscript).

Use urwid's calc_width in make_canvas

This does the right thing for double-width and zero-width Unicode characters.

Fixes #232.

Tell ptpython to read/process the user's config file.

Add support for custom shells

Miscellaneous code cleanups

The argument order globals, locals matches eval and exec. The first_time
argument is unnecessary, and just clutters the API.

A custom shell can be added in a similar way to a custom stringifier. Create a
file with a function pudb_shell(_globals, _locals, first_time) at the module
level, and set it in the settings.

The first_time setting is there to match the existing shells, but it seems
useless (a function can easily tell if it has already been called), so it
should perhaps be removed.