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
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
Merge pull request #426 from aterrel/google-analytics

Add google analytics to docs
expr: selection doesn't fail on non-rowwise child

Effect is that nested selections or really any selection of a non-rowwise
is still acceptable.

Thanks @talumbau for reporting