Update Entity class to be compatible with the new Dataset entity

[vpchung]

One of the fields of the Dataset data model is named, items, which coincides with a Python builtin: dict.items(). This results in a TypeError when trying to print a representation of the entity:

>>> dataset = syn.get("syn123")
>>> repr(dataset)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/vchung/Desktop/synapsePythonClient/synapseclient/entity.py", line 434, in __repr__
    self.properties.items(),
TypeError: 'list' object is not callable
>>> 

Looking at the Entity class, the __repr__() should be updated as to not call on self.properties.items(), as to not confuse Python what it is we want (key-value pairs of self.properties vs. the actual property named items).