expr: add Union operation
pandas: union
python: union
spark: union
sql: union
DataDescriptor.py[...] -> DataDescriptor[...]

Issue: this promotes python types as the default itemaccess type
Benefit: .py was confusing because it looked like a file extension
dd.py[...] -> dd[...] for compute(Projection, DD)
Add google analytics to docs
add pytables compute draft
small edits to pytables
numpy: fall back on pandas for by, join, ...
try to import pytables
fix pip install datashape command
pytables: compute column
add pytables testing file
add pytables to travis testing
Speed up import by turning off multipledispatch ordering

See https://github.com/mrocklin/multipledispatch/pull/29

This significantly speeds up import time.  Dispatch import time used to
dominate (4s if using all backends) now it's a small fraction of a second.
Merge pull request #442 from mrocklin/fast-import

Speed up import by turning off multipledispatch ordering
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