This includes most API methods on pull requests (with the exception of
listing commits and the merge functions), as well as all of the methods
for pull request comments. (Fixes #13)

This also fixes a few oversights in the issues API.

This adds basic methods for working with issues, assignees, and issue
comments.  Issue events, labels, and milestones are not included in this
commit. (Fixes #7)

golint doesn't like the underscore in "url_"

This allows for making unauthenticated requests subject to your
application's rate limit (as opposed to the lower rate limit for
anonymous requests)

if we have an error reading the body of an API error response, still
return an ErrorResposne with the original response struct.  Simply
returning a string stating that we couldn't read the response body
doesn't give the caller anything actionable to work with or attempt to
process.

This now aligns with the (pretty strong) golang convention of the
package name matching the path basename

this is particularly important for methods like
Organizations.CheckMembership which return a 404 to indicate a
successful response of "false".

also minor doc cleanup

This commit adds the core library funcationality and establishes the
general calling style and testing structure.  Only a few GitHub API
methods related to organizations and repositories are included in this
first commit, mainly to cement the calling style.