[SPARK-6379][SQL] Support a functon to call user-defined functions registered in SQLContext

This is useful for using pre-defined UDFs in SQLContext;

val df = Seq(("id1", 1), ("id2", 4), ("id3", 5)).toDF("id", "value")
val sqlctx = df.sqlContext
sqlctx.udf.register("simpleUdf", (v: Int) => v * v)
df.select($"id", sqlctx.callUdf("simpleUdf", $"value"))

Author: Takeshi YAMAMURO <linguin.m.s@gmail.com>

Closes #5061 from maropu/SupportUDFConversionInSparkContext and squashes the following commits:

f858aff [Takeshi YAMAMURO] Move the function into functions.scala
afd0380 [Takeshi YAMAMURO] Add a return type of callUDF
599b76c [Takeshi YAMAMURO] Remove the implicit conversion and add SqlContext#callUdf
8b56f10 [Takeshi YAMAMURO] Support an implicit conversion from udf"name" to an UDF defined in SQLContext