Use by default SSL CA certificate bundle from the platform

If pyopenssl is available and you give an empty ca_certs to urllib3,
it will use the default CA of the current platform.

Not ready for merging as this feature requires a bleeding edge version of urllib3 (https://github.com/shazow/urllib3/commit/5c25a73dfb48e4260c44e19e3a50fb5d46832c52)

I post this PR now, to discuss about its implementation and testing improvements.
About testing we could run the project with tox and iterate among different versions of python and with or without dependencies of urllib3.contrib.pyopenssl.
```
pyOpenSSL
ndg-httpsclient
pyasn1
```
thx