add doc section on conditional requests
add link to api docs for conditional requests
fix copy/paste typo in conditional request docs
add missing fields for various types

Includes additions to User, Organization, Repository, and Gist.  Some
minor changes to existing fields based on changes in the v3 API (for
example, Gist.User became Gist.Owner).  Most everything else is new
additions.

refs #103
Added support for the GitHub Pages API.
Fix English sentence
RepositoriesService.GetByID
UsersService.GetByID