various improvements to drugbank parser

* give up on difflib - results not great
* deal with HMSL compounds with same id, different salt
* parse drugbank brand names as synonyms
speedup by switching from engine to connection/transaction

I originally used the sqlalchemy 'engine' object to execute all the queries
without realizing this was opening/closing a connection each time, and was
terribly slow. The performance hit only showed up when I switched from :memory:
to an actual file. This commit changes to using a persistent connection, and
furthermore wraps each batch of inserts/updates in a transaction.
reuse existing drugbank data in sqlite if present
ignore drugbank sqlite files
store matches in a table
Merge remote-tracking branch 'gberriz/master'
map lxml.etree._ElementStringResult to string
Merge remote-tracking branch 'gberriz/master'
Merge branch 'master' of github.com:jmuhlich/lincs-drug-targets