Basic setup for tests

Based on the test setup of stub.sh: https://github.com/jimeh/stub.sh
Add tests for lib/env.sh
Improve global test setup in test-helper.sh
Fix path related test setup issue
Add tests for lib/runtime.sh
A few further tweaks to how path handling in tests
Add tests for lib/util.sh
Add basic tests for tmuxifier-tmux
Colorize part of test output
Use tput instead of hard-coded color codes
Install tmux on travis-ci.org
Attempt to get tmux 1.8 on Travis-CI boxes
Attempt to run travis-ci tests against multiple versions of tmux
Attempt to fix test failing with Tmux 1.6 and 1.7
Add slight speed improvements for Travis-CI
Update stub.sh
Improve test environment for individual test files
Add tests for __expand_path layout helper
Add tests for __go_to_session layout helper
Use test-runner.sh instead of homegrown test.sh