[SPARK-7394][SQL] Add Pandas style cast (astype)

Author: kaka1992 <kaka_1992@163.com>

Closes #6313 from kaka1992/astype and squashes the following commits:

73dfd0b [kaka1992] [SPARK-7394] Add Pandas style cast (astype)
ad8feb2 [kaka1992] [SPARK-7394] Add Pandas style cast (astype)
4f328b7 [kaka1992] [SPARK-7394] Add Pandas style cast (astype)

python/pyspark/sql/column.py

@@ -302,6 +302,8 @@ class Column(object):
             raise TypeError("unexpected type: %s" % type(dataType))
         return Column(jc)
 
+    astype = cast
+
     @ignore_unicode_prefix
     @since(1.3)
     def between(self, lowerBound, upperBound):