Implement Issues Events API

Fixes #111.  Thank you guys for making it easy to get started with this project!  This is my first real open source pull request:)

Please take a look at the response objects EventInfo and IssueEvent as I wasn't thrilled with them.  I ran into a json serialization issue so I pulled EventInfo out of IssueEvent.  I could of made everything an IssueEvent but then didn't like Issue always being null for one of the client methods.

Please let me know if there is anything I can do to help.