Add ChefSpec::Cacher module for faster specs

The cacher module allows for ultra-fast tests by caching the results of a
Chef Client Run in memory across an example group. In testing, this can
reduce the total testing time by a factor of 10x. This strategy is _not_ the
default behavior, because it has implications surrounding stubbing and is
_not_ threadsafe!

- Refs: #275
- Refs: http://dracoater.blogspot.com/2013/12/testing-chef-cookbooks-part-25-speeding.html
- Credits: @DracoAter
Only test the master branch on Travis
Add Media & Third-Party Tutorials [ci-skip]
Add Travis IRC notifications
Allow fast finishing on Travis
Automatically configure RSpec if cacher is required

Why would be make users do more work than they need to! Turn on the magic!
Requiring 'chefspec/cacher' automatically extends RSpec with the +cache+
method for you!