[SPARK-6603] [PySpark] [SQL] add SQLContext.udf and deprecate inferSchema() and applySchema

This PR create an alias for `registerFunction` as `udf.register`, to be consistent with Scala API.

It also deprecated inferSchema() and applySchema(), show an warning for them.

cc rxin

Author: Davies Liu <davies@databricks.com>

Closes #5273 from davies/udf and squashes the following commits:

476e947 [Davies Liu] address comments
c096fdb [Davies Liu] add SQLContext.udf and deprecate inferSchema() and applySchema