improved test descriptions
refactoring MRBrewFormula with improved api, breaking backwards compatibility

- the properties 'updated', 'newFormula' and 'installed' now have more consistent accessor method names (isUpdated, isNew, isInstalled...)
- replaced initWithName:newFormula with initWithName:isNew:isUpdated:isInstalled:
- replaced formulaWithName:newFormula: with formulaWithName:isNew:isUpdated:isInstalled: