release-0.2.0

• Reworked the instance launch method to require subnet vs. network. This
  removed the option of adding network interface to a launch config object.
• Added object store methods: upload from file path, list objects with a
  prefix, check if an object exists, (AWS only) get an accessible URL for an
  object (thanks @VJalili)
• Modified get_ec2_credentials() method to get_or_create_ec2_credentials()
• Added an option to read provider config values from a file
  (~/.cloudbridge or /etc/cloudbridge)
• Replaced py35 with py36 for running tests
• Added logging configuration for the library
• General documentation updates