TTTTimeIntervalFormatter's precision can be specified

Adds `numberOfSignificantUnitsToDisplay` and `leastSignificantUnitToShow` properties.
Adds assertion in setLeastSignificantUnitToShow:
Better naming consistency.
AppleDoc comment style more consistent with rest of project.
Merge pull request #51 from defragged/specify-precision

TTTTimeIntervalFormatter: Specifying Precision