add some comments
move nirvana mode functions into their own space

This commit moves nirvana mode functions out of the top-level 'waylon'
space and into their own 'waylon.nirvana.*' space.
fix css class issue when build status changes

Prior to this commit, when a job's status changed (e.g. from success to
building), waylon.js would append CSS classes to the element. This would
leave us with something like:

    <tr class="successful-job building-job" ...

instead of the desired

    <tr class="building-job" ...