Pdblp

- Fix bopen() to work with passed session
- Refactor pdblp to use blpapi.Messages parsed into dicts
- Remove conda package building
- Remove testing for python 3.5 and 3.4

- Automate conda and pypi deploys with TravisCI
- Change from unittest to using pytest
- Seperate offline tests which do not require BBG connection
- Refactor event handling and logging into _receive_events()
- Remove restart() method, see issue 46
- Allow constructor to take a blpapi.Session and blpapi.Identity, for SAPI

- Add ability to chain start() method, e.g. con = BCon().start()
- Fix logger to interface with upstream logging libraries
- Add pull request template
- Add NaN support to parser grammar
- Support empty bulkref() requests

- Seperate ref() into bulkref() and ref(), see issue 35
- Update tutorial docs to reflect new functions
- Update Sphinx build process to use sphinx 1.7.5 and provide environment
file for building docs

- Fix bug accessing null blpapi.element.Element in ref(), see issue 32
- Raise a ValueError for bad inputs to bdh(), see issue 33

- expose blpapi.Event.TIMEOUT to user through timeout parameter in BCon()
instantiation