Make SSL cert verification globally configurable

When debugging using test servers, self signed certificates
are often used for HTTPS. This commit adds a configuration
option that disables SSL certificate verification globally
for all requests made from the mist.io app to the hosted
core service.

The config.SSL_VERIFY option is True by default but can be
overriden in settings.py.
Catch requests.exceptions.SSLError

and raise mist.io.exceptions.SSLError so that it is properly handled
by our exception handler and transformed to an appropriate HTTP
response.

Also deprecate code that authenticates to core service from __init__,
that's handled in check_auth view.
Merge pull request #77 from mistio/ssl_verify

Ssl verify