Provide a high level description of testing within puppet
Add documentation on examples
Add a section on example groups
Document before and after blocks, let blocks, mocks, and stubs
Merge branch 'adrienthebo-doc-introduction_to_rspec'

* adrienthebo-doc-introduction_to_rspec:
  Document before and after blocks, let blocks, mocks, and stubs
  Add a section on example groups
  Add documentation on examples
  Provide a high level description of testing within puppet

closes #1592
[ci skip]