[SPARK-6553] [pyspark] Support functools.partial as UDF

Use `f.__repr__()` instead of `f.__name__` when instantiating `UserDefinedFunction`s, so `functools.partial`s may be used.

Author: ksonj <kson@siberie.de>

Closes #5206 from ksonj/partials and squashes the following commits:

ea66f3d [ksonj] Inserted blank lines for PEP8 compliance
d81b02b [ksonj] added tests for udf with partial function and callable object
2c76100 [ksonj] Makes UDFs work with all types of callables
b814a12 [ksonj] support functools.partial as udf

(cherry picked from commit 98f72dfc)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>