SgFieldImage.uploadFilmstripThumbnail possible error if parent is None

Swaped to if statments due to parent possibly being None.
SgConnection added baseEntityClasses function and init arg

Its not easier to specify custom Entity base classes for connections
by passing an arg to the constructor and for SgConnection
subclasses defining SgConnection.baseEntityClasses() returning a
dict.

The constructor arg and return result of baseEntityClasses are
merged with the constructor arg overriding any values of
baseEntityClasses.