adding the ability to login with different dockercfg files.

flake8 fails with F821 for unicode under python 3.4.2

Tidy up stream test harness & fix race conditions

Conflicts:
	tests/test.py

Support for host config

* Warnings will now raise exceptions in unit tests
* Updated several tests to use host_config

* Deprecation warnings added in Client.start

Devices should be separated by colon separated string

Run Travis tests on Docker

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

Totally bad test :( (written by me). The documentation and
code should match now. Many thanks to pranavs18 for spotting
the error and proposing the fix.

Data already in local python buffers was lost when stream socket
fileobject was re-created. We now use http.client to handle the chunk
transfer encoding, and we read only the data from each chunk.

This adds a test harness for stream responses which tries to trigger the
lost-buffer behaviour by responding to the HTTP request in full, as
early as possible.

NB: Python's http.client will close the socket fileobj early if there is
no content length or chunked transfer encoding header. If this happens,
requests/urllib3 will reopen it, but we lose some data which was stored
in buffers.

Fixed bug in utils.mkbuildcontext for BytesIO Dockerfiles