requirements.txt needs python-dateutil less than 2.0

The requirements.txt file should require the packages necessary to install
Flask-Restless regardless of platform. The setup.py file should be smarter
and require only the necessary platform-dependent packages.
check all parent classes for hybrid properties

In order to find all hybrid properties, the code must look through all base classes. This is because the object of type `hybrid_property` is only visible in the `__dict__` of the base class where it was defined, and not in derived classes. Without this, if you have a `hybrid_property` defined in a base class (perhaps one containing several common columns and a property calculation that you'll need on more than one table) then this code wouldn't find that `hybrid_property` to show it in the API.

For more on `hybrid_property`, see also pull request #138.
Merge pull request #156 from glyphobet/hybrid-properties-on-base-classes

check all parent classes for hybrid properties