add support for disabling the timestamp in build version

If the OMNIBUS_APPEND_TIMESTAMP environment variable is unset or set to 
a truthy value we will append a timestamp to the build version.  This 
behavior can be overridden by setting this value to a "falsey" value 
(i.e. false, f, no, n or 0).
Expose a 'real' omnibus CLI!

A real grown-up CLI for Omnibus projects! Initially this will be used to
provide a "build" subcommand with "--no-timestamp" option which will be 
used in CI for release jobs.

Future enhancements include:

* A "create" subcommand that will allow easy skeletal project generation
* Possible elimination of Rakefile
* OMG! Ponies!
depend on latest stable Thor 0.17.0
Break-up method_option invocations.
use Mixlib::ShellOut to spawn rake process
Add a .gitignore file and remove the Gemfile.lock

It is an snit-pattern to check in the Gemfile.lock in gem repositories.
Merge branch 'cli'
Bump version to 0.5