Demisto-py

* Added `ssl_ca_cert` configuration option to specify an alternate certificate bundle.
* Added support for additional configuration environment variables:
* `DEMISTO_VERIFY_SSL`
* `SSL_CERT_FILE`

* Added `investigation_add_entries_sync` method creating a new entry in existing investigation.
* Added `download_file` method for downloading files by entry id.
* Added `integration_upload` method for uploading integrations to Demisto.

Added support for user/password authentication option.

Removed unsupported user/password authentication option.

Fixed missing dependency (tzlocal).

* Fixed issue in `generic_request` where body was being ignored ([20](https://github.com/demisto/demisto-py/issues/20)) .
* Added `content_type` and `accept` parameters to `generic_request`.
* Fixed `datetime` object formatting to include timezone offset as specified in RFC 3339.