utils: add kwargs_from_env

Fixes #359

Usage:
```
proppy@noir:~/docker-py$ $(boot2docker shellinit)
Writing /home/proppy/.boot2docker/certs/boot2docker-vm/ca.pem
Writing /home/proppy/.boot2docker/certs/boot2docker-vm/cert.pem
Writing /home/proppy/.boot2docker/certs/boot2docker-vm/key.pem
proppy@noir:~/docker-py$ python <<EOF
> from docker.client import Client
from docker.utils import kwargs_from_env

client = Client(**kwargs_from_env(assert_hostname=False))
print client.version()

> > > > > > EOF
{u'KernelVersion': u'3.16.4-tinycore64', u'Arch': u'amd64', u'ApiVersion': u'1.15', u'Version': u'1.3.0', u'GitCommit': u'c78088f', u'Os': u'linux', u'GoVersion': u'go1.3.3'}
```