Track build event channels per project rather than change

Since we'll be having a fairly static/limited number of projects
we'll also have a fairly static number of channels tracking build
jobs.

Fixes #3.

Change-Id: I441114e396649a5f9f930c9bee26c602d13f508d
Use break to terminate the select, not return.

Fixes #4 based on discussion at:

  * http://stackoverflow.com/questions/11104085/in-go-does-a-break-statement-break-from-a-switch-select
  * http://golang.org/ref/spec#Break_statements

Change-Id: I429542def1b7615c03ef18d0228aaac75c8c0d2f