The prompt_choices function now returns a correct result.

The prompt_choices function would try to call string.ascii_lowercase(rv) where
rv is a string.
This would raise an error because ascii_lowercase is not a function. Assuming
that the intended result is a lowercase version of the string, this has been
changed to use rv.lower()

Also the check::
        if isinstance(string, basestring):
has been changed to::
        if type(string) is str:

because basestring is not defined in python3.
A check has been added to know if args has an attribute 'callback'.

If args has no attribute callback, the help text should be printed. Because
there was a check to::
        if args.callback is 'x':
It would raise an error before the parser.print_help() was reached.
Merge pull request #48 from JorisE/master

Fixed two issues in cli
Try adding space before Pane.send_keys commands so shell doesn't record a command in history.
v0.1.3
remove comment for old --log-level argument.  Use isinstance of string_types for checking choices in prompt_choices.
coding: utf8 fix space.
v0.1.4. Update changelog.
Fix link in CHANGES.
docs: Section headings.
docs: doc/index.rst reference now _index
CHANGES updates.
Add package_metadata from tony/cookiecutter-pypackage. Update setup.py and doc/conf.py
v0.1.5
Fix #49 package_metadata.py missing from MANIFEST.in
Update link in CHANGES.
Try to test removing self.server.tmux for default-path on new_window.
Try using -c start_directory in Session.new_window, remove the hack where we set-option default-path.
Update to show error params on new_window creation of builder.
Join list when raising error.