expr: add Union operation
pandas: union
python: union
spark: union
sql: union
Merge branch 'master' into union

Conflicts:
	blaze/compute/python.py
	blaze/compute/tests/test_pandas_compute.py
	blaze/compute/tests/test_python_compute.py
	blaze/compute/tests/test_spark.py
	blaze/compute/tests/test_sql_compute.py
	blaze/expr/tests/test_table.py
Merge pull request #399 from mrocklin/union

Union operation