Change Method Names and Add Get Collection

dataverse.py
-add getStudies which creates a list of study objects from the Atom
feed that is returned when you GET a collection
-add addStudyToFile() which uses the append method. this does not work
yet on the server side.
-add the replaceStudyContents() method, which replaces any files in the
study with the contents of the zip package passed to it.

dvn_client.py
-add getStudies() call

study.py
-refactor class methods for creating studies
-add __repr__ for debugging
-add CreateStudyFromEntryElement that creates a Study object from the
Element object that is the result of parsing the Atom feed with lxml