Fixing openssl command example in SSL guide

Fixes #138
Renamed EP example to HelloEndpoint; Ruby 1.9 syntax; minor wording tweaks
Added missing commas so guides would compile
Example #2 of the endpoint creation tutorial
Example 3 of endpoint generation tutorial - finit!
Link to hello_endpoint github repo
Add navigation sidebar for messages
Add placeholder pages for message documentation
Sidebar navigation for endpoints; placeholder for Mandrill endpoint
Finished first draft of Notification Messages guide
Further progress on the mandrill_endpoint guide.
Revised the navigation of integration guides
Started a new Terminology guide

Ported over some content from the legacy Overview guide and made some changes.
Add Failures to Terminology guide
Fixed typo in integration sidebar navigation.
Explain Service, Service Request and Service Response
Add $ to bash commands
Use bundle exec rackup
Update code blocks to follow style guide
Change order message headings