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.

This results in a more complete and readable timeline.

Populate it from Gerrit CLs. It's helpful to see when
new patch sets have been uploaded in between comments.

It's not supported by other change.Service implementations
yet, but that can be done when there's some demand for it.

These events have been superseded by Review and its State field.

This check is now performed upstream in maintner,
so no need to do it here.

Follows https://golang.org/cl/107296.
Updates https://golang.org/issue/21984, https://golang.org/issue/22060, https://golang.org/issue/24875.

Similar to https://github.com/shurcooL/issues/commit/697ee8e877735feed6151525cad859327bfe40c4.

Follows https://dmitri.shuralyov.com/state/...$commit/28bcc343414c6adcd7b6911f9d0ef1ad6fbf30ae
and https://github.com/shurcooL/events/commit/25a0212309664cda69930a58f8369f24b9029812.

Change timeline item ID type from uint64 to string.

Rearrange services.

There's no longer a clean distinction between comments and events.
There are other timeline item types that don't fit into either category
cleanly. Also, underlying implementations for various APIs are very
similar and have overlap. As a result, it makes sense to merge these
two methods into one.

Remove ListCommitsOptions from ListCommits. Expected usage is to always
get all commits, then find the one you're interested in. This is what's
needed to implement prev/next commit buttons, and more closely matches
what's returned by 3rd party APIs.

The button styling can be improved; to be done later.

Add GetDiff method to it.