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.
de
Merge pull request #55 from BlackLee/master

Add a percent symbol for German
Merge pull request #51 from defragged/specify-precision

TTTTimeIntervalFormatter: Specifying Precision
Improving organization of docs headers
[Issue #51] Dropping 'ToDisplay' from new properties' names
Bumping version to 1.2.0
Removing unnecessary validations