Make the BSD 3-Clause license easier to see by adding a LICENSE file
to the repository. Use the same format as https://golang.org/LICENSE.

These methods will be used to produce links to issue and PR events.

Split IssueURL into IssueURL and IssueCommentURL.

Split PullRequestURL into PullRequestURL and PullRequestCommentURL.

Add PullRequestReviewURL and PullRequestReviewCommentURL.

Factor out Router from github.com/shurcooL/notifications/githubapi
package, so it can be more easily imported both by notifications/githubapi
and events/githubapi.

Add context parameter to enable some request-scoped flexibility over
routing.