Show Count of Attached Tmux Sessions

- This segment displays the number of attached tmux clients to the
  currently running session.
- The minimum argument is used to specify a threshold for when the
  segment should be visible.

Fixes #661
Closes #662

Conflicts:
	docs/source/index.rst
	powerline/config_files/colorschemes/shell/default.json
	powerline/config_files/colorschemes/shell/solarized.json
	powerline/config_files/colorschemes/tmux/default.json
	powerline/config_files/colorschemes/vim/default.json
	powerline/config_files/colorschemes/vim/solarized.json
	powerline/config_files/colorschemes/wm/default.json
	tests/test_segments.py
Move some functions to from bindings.config to bindings.tmux
Make attached_clients segment use provided tmux bindings
Wrap docstring and add dot
Add __future__ import