Add build-log plugin

[danielbeardsley]

I can see a use for two plugins:

build-log

• Outputs a running log of build events (like the server log is now)
• Use a defined format
  • timestamp: build_id repo branch status

build-status-log

• Updates a JSON file after every build, keeping the last 3 or for builds for every branch.

  • Effectively used as a data store (bad idea?) (read in on load, written out after every build.

  • Makes it very easy to write command-line or web-based tools.

  • Contents:

    { 'http://repo-url' : { 'branch': [ {buildid: 23, started: 34626326, status: 'success', logUrl: 'http:///blah.com/log', buildTime: 3453}, {buildid: 26, started: 34626326, ...}], 'other-branch': [ {...}] }}