Add entry on callbacks

Uses the examples of callbacks as blocks in Sinatra request handlers and
an event listener in the browser
Change entry title callbacks to callback (singular)
Edit entry on callbacks to focus on smaller examples

Omit references to events or nonlinear programming. Place priority
focus on callbacks as functions passed to other functions, using
iterative functions as an example.

Expand upon definition to show how callbacks can be used for delayed
execution.
Merge pull request #3 from codeunion/add-entry-on-callbacks

Add entry on callbacks