refactor timecode module

Refactor function `generatePart* to `ts2tc`.

* add docblock
* add parameter to add milliseconds to timestamp explicitly
* optimise function body for readability.

Add convenience method to convert a single timestamp (float) to a
timecode string.
refactor progressbar module

Use tailored tc.fromTimeStamp instead of tc.generate.

Refactor update method.
It was part of the Progressbar prototype and is now just a private
function `_update`. As it is used as an event listener it must be bound
to the Progressbar instance anyways.
refactor ProgressBar

ProgressBar module renders the entire progress bar element.
Progress bar element has only a class “progressbar”.
Added function renderTimeElement.
Remove all id attributes. Keep references to progress and currentTime
elements to alter them in update function
Merge pull request #1 from line-o/progressbar

Progressbar