Platform ca location

WIP do not merge yet.

@shazow I'm not sure I understood properly your advice, if not, I'm willing to re-implement it.

For the tests. I decided to perform a real connection to `https://travis-ci.org/` . If it is not acceptable
I see one alternative, that can be performed on CI environment.

I can add https://github.com/shazow/urllib3/blob/master/dummyserver/certs/cacert.pem to list of
root-ca of the system. For ubuntu (travis-ci.org ) it is like

```bash
$ cp dummyserver/certs/cacert.pem /usr/local/share/ca-certificates/
$ update-ca-certificates
```

But this change will break tests that check https connections without a specified ca_certs raises `SSLError`, because `/etc/ssl/certs/ca-certificates.crt` will be used by default.
Not mentioning that modifying global ca-certs of the system of developers who wants to run the tests, is probably not acceptable.

 Unless you see a third option, I think it is for the best like this.