Work on correct grid-labels

GridLabels require absolute start date of recording - added it into unit converter.

Added tests to the unit converter and updated all usages of it.

Also refactored Media creation in listen so that it properly transforms its value.
Then fixed associated bugs with Media model (sample rate was set on the resource not on this)
	and added appropriate unit tests.
Grid line labels corrected for Annotation Library
Merge pull request #131 from QutBioacoustics/grid-labels

fix(gridLines) Fix grid labels, Fixes #121