Added phpunit/skel-gen to composer dev dependencies.
Fix #66 : Add a method to PropertyBuilder to push() a Property.
Created a new interface to represent a PropertyContainer.
VCard now implements that interface.
Also added a trivial concrete PropertyContainer as PropertyContainerImpl.
Added a method to PropertyBuilder to build() and push() to a PropertyContainer.

Also added a clear() method to PropertyContainer and thus VCard,a nd modified
push(..) to take variable arguments and to accept a PropertyContainer to
push multiple properties in one call.

Added some testcases.