Adding `proxies` and `verify` parameters for TAXII Client

[thelok]

Adding proxies and verify parameters to the attack_api.py constructor to programmatically define proxy information. These params will be passed to the TAXII client.

proxies - See https://requests.readthedocs.io/en/latest/user/advanced/#proxies verify - See https://requests.readthedocs.io/en/latest/user/advanced/#ssl-cert-verification

These changes should be transparent to those that don't need to use custom proxy or SSL verify functionality.

Fixes #56