TLS support, with same env vars as docker client

Thanks to @jkingyens for the bulk of the work.

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>
Vendor dockerpty at c8b493553477c9a57d163c71c97b2102f44a6ce7

Include TLS fixes and relative imports:

https://github.com/aanand/dockerpty/commit/c8b493553477c9a57d163c71c97b2102f44a6ce7

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>
Document TLS environment variables

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>
Update docker-py to 0.5.2

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>
Update install guide to use $(boot2docker shellinit)

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>