Add example for execute_command fluent interface.
Ignore artifacts generated by bundler.

...for example when installing the gems within the project dir:

  `bundle install --path vendor --binstubs`
Update execute_command scenario to fail.
Add `with' method for execute_command matcher.
Update README and scenarios for execute_command.

The new method doesn't work with the `only_if` or `not_if` attributes of
a command resource (cucumber just blew up), so I just updated the doc
and scenarios to use different attributes.

  (see also https://github.com/acrmp/chefspec/pull/35)
Refactor execute_command matcher and specs.

...and switch to the new RSpec syntax, Yay!
Merge pull request #103 from bjoernalbers/execute_command_with

Add fluent interface to execute_command matcher