Add functions for finding Deal and Task categories.

New functions added to the HighriseAPI class:
  - public findAllDealCategories()
  - public findAllTaskCategories()
  - protected findAllCategories()

New classes:
  - HighriseCategory
  - HighriseDealCategory
  - HighriseTaskCategory

Example usage:
    $deal_categories = $highrise->findAllDealCategories();
    $example_category = $deal_categories['Example']->getId();

    $deal = new HighriseDeal($highrise);
    $deal->setName('Example Deal');
    $deal->setCategoryId($example_category);
    $deal->save();

A test is included in test/categories.test.php.
Correct typos with Exception calls