Add Report state change, assignee update, and comment functionality

[bored-engineer]

HackerOne previously added endpoints for creating comments, updating a report's assignee, and updating a report's state:

September 21st, 2016: added ability to assign users and groups to a report.
November 2nd, 2016: added ability to change the state of a report object and added ability to post internal and public comments.

This pull request adds support for these endpoints via additional methods on the ReportService struct. Since there are the first state-changing endpoints with POST bodies changes were made to h1.go to add support for request bodies. Additionally this was the first time a struct (Activity) had to be JSON marshalled so some changes have been made there. I'm open to suggestions about the best method to do this since I'm not thrilled with the current implementation but it does work...

[bored-engineer]

This fixes #9, #10, and #11.

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