Merge latest changes from dev into master

[richardwu]

We need to update master with dev now that dev is stable. Changes in this merge:

• Fixed numerous exception handling bugs
• Normalization of clean and training data is now consistent
• Added Travis CI
• Added a unit/integration test
• Fixed a couple of typos in README
• Added create_db_ubuntu.sh and create_pd_user_ubuntu.sh scripts for initializing postgres
• Added numerous docstrings and comments to functions
• Introduced a posterior probability estimator to approximate the conditional distribution of values for weak labelling and domain generation. This improves weak supervised learning. See parameters weak_label_thresh, domain_prune_thresh, and max_domain (#43)
• Outputting weights for featurizers when print_fw=True
• Disable pool creation when threads <= 1
• Fixed UTF-8 encoding issue when ingesting training data
• Fixed bug where non-lowercase attribute names caused Postgres to fail
• Added two new test datasets met and Adult

[jonmio]

Left some comments. Mostly style related and some documentation fixes. Also, left a few questions.