Allow passing custom dns_resolver to client.connect

[daurnimator]

By passing a custom dns_resolver object you can:

• Use a custom nameserver
• Specify host=>ip mapping manually using a cqueues dns.hosts object

It has been written as a base for connection pooling (#10) and Happy Eyeballs.

[coveralls]

Coverage increased (+0.2%) to 86.9% when pulling 5a80e99459f4f893bfbfaa324a9fb08bcaedf116 on client.dns_resolver into e7cb0769ea75655a04b9b32de9c66bf1df99a401 on master.

[daurnimator]

The http.socks module should gain a similar ability.