Expose setting net/http SSL verify_callback.

This is helpful for users wanting to apply their own custom certificate
verification logic. Add warnings by default if it is used on OS X or
jruby, which may not support the functionality as expected.
Add some tests around ssl_verify_callback.
Fix tests, verify_callback functionality on jruby.
Merge pull request #277 from ab/ab-ssl-verify-callback

Expose verify_callback to users