Added load from file object functionality to pyassimp.load

Changes so that when pyassimp.load is called, we check to see if it is a file object, and if so load the data from it (as opposed to just being able to load paths). There is an additional keyword argument, 'file_type', which must be specified if file objects are passed. Example, where both load the same mesh:

s = pyassimp.load('featuretype.STL')
s = pyassimp.load(open('featuretype.STL', 'rb'), file_type='stl')

The changes to helper.py are to load the aiImportFileFromMemory function (and minor cleanup).