tmux: Colorize the last active window FG

Use a feature of tmux v1.8+ to change the FG color of the last active
window to blue. This uses the foo-{attr,bg,fg} format, which is
deprecated (although still available) starting with tmux v1.9 in favor
of the corresponding foo-style variable. However, tmux v1.9 is not yet
ubiquitous enough to move to the foo-style variables. For example, the
latest Ubuntu LTS (14.04) only has tmux v1.8 available in its
repositories, so the foo-{attr,bg,fg} variables prove to be more
'compatible' for now.

For future reference, here is the modification that should be made once
tmux v1.8 is deemed old enough to drop support for (or whenever the
deprecated foo-{attr,bg,fg} variables go away in the latest version of
tmux):

-set -g window-status-last-fg colour31
+set -g window-status-last-style fg=colour31

It might be appropriate to add version checking around some of these
types of options for maximum compatibility, but my initial attempt to
explore version checking in the tmux config file got extremely messy in
a hurry.
Fix shutdown function being always None

Problem reported by @IvanMalison
Undo renaming args to kwargs
Merge pull request #867 from ZyX-I/fix-shutdown

Fix shutdown function being always None
Merge pull request #864 from austinbeam/feature/tmux-colorize-last-window

tmux: Colorize the last active window FG
Merge pull request #863 from austinbeam/feature/tmux-indicate-prefix

tmux: Change session block color if prefix pressed
tmux: Shell variable definition for tmux version

Add variables for tmux major and minor version numbers. These can
subsequently be used to test for tmux version compatibility.
tmux: Check version for prefix active colorization

Prefix active colorization feature introduced by pull request #863
causes undesired behavior on older versions of tmux. Add version
checking and revert to old powerline configuration if the tmux version
is less than v1.8. tmux v1.8 was the first version to support the
client_prefix format variable.
tmux: Check version for last window indication

Last active window indication feature introduced by pull request #864
causes undesired behavior on older versions of tmux. Add version
checking to determine which method for feature implementation to use,
dropping the feature entirely for tmux versions less that v1.8. For tmux
v1.9, use the newer 'window-status-last-style' option rather than the
deprecated (starting with v1.9) 'window-status-last-fg' option.