Stay consistent with absolute imports
Use methods= as kwarg

No functional change, just for readability.
Support default value for EnvironmentVariables

If a default_value is specified in the config
tuple for EnvironmentVariables for 'config_file', it wasn't
being honored by the Session object.  This update will return
a default value if a non None default is provided and if the
calculuated value is None.
Add default value for aws config file

To use a config file to store credentials, you first
had to create a file, and then set the AWS_CONFIG_FILE env var
to this file's locations.  Now a default location of ~/.aws/config
is checked if AWS_CONFIG_FILE is not set.
Add additional docs about the tuple params
Add test to ensure env var beats config defaults
Merge branch 'config-refactor' into develop

Closes #39.

* config-refactor:
  Add test to ensure env var beats config defaults
  Add additional docs about the tuple params
  Add default value for aws config file
  Support default value for EnvironmentVariables
  Use methods= as kwarg
  Stay consistent with absolute imports