Support for input variable JSS_VERIFY_SSL

In my test instance, I have a self-signed cert. While I might be able to convince Requests that the cert is trusted in OS X's keychain, I would imagine that a server depending on SNI would also have issues (given Python's SSL support in 2.7 being still lacking on OS X). Requests still cannot do SNI unless additional SSL libraries are provided, according to their docs.

Anyway, to alleviate these issues I added a simple boolean pref that supports passing the appropriate option to your jss module's `ssl_verify` function arg for `JSS()`. Defaults to True, of course, if it's not specified.