Thread safe AWS account name resolution

[s-maj]

When --resolve-aliases is used, it is possible to receive (from time to time) exception:

Traceback (most recent call last):
  File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.6/site-packages/aws_google_auth/amazon.py", line 76, in resolve_aws_alias
    sts = boto3.client('sts', region_name=self.config.region)
  File "/usr/local/lib/python3.6/site-packages/boto3/__init__.py", line 83, in client
    return _get_default_session().client(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/boto3/session.py", line 263, in client
    aws_session_token=aws_session_token, config=config)
  File "/usr/local/lib/python3.6/site-packages/botocore/session.py", line 851, in create_client
    endpoint_resolver = self.get_component('endpoint_resolver')
  File "/usr/local/lib/python3.6/site-packages/botocore/session.py", line 726, in get_component
    return self._components.get_component(name)
  File "/usr/local/lib/python3.6/site-packages/botocore/session.py", line 926, in get_component
    del self._deferred[name]
KeyError: 'endpoint_resolver'

This issue is related to boto/boto3#801

One session per thread should fix this.

[coveralls]

Coverage increased (+0.003%) to 49.068% when pulling b075b1c on s-maj:threadsafe into 9ffde70 on cevoaustralia:master.

[stevemac007]

Looks good, thanks for the contribution.

[stevemac007]

Will merge and release new version shortly.