Delete contact method

[kbairak]

This adds a method for deleting a contact on hubspot.

Unfortunately, there's no easy way to test this. I had to try this out on our production environment by copy pasting commands into a python shell and having someone with access to hubspot verify that it worked. The snippet I used was:

from django.conf import settings

from transifex.libs.analytics import get_hubspot_contact_proxy

h = get_hubspot_contact_proxy()

# The arguments of the new delete method
email = "..."
api_key = settings.HUBSPOT_SETTINGS['api_key']

# The body of the new delete method
contact = h.get_contact_by_email(email, api_key)
vid = contact['vid']
subpath = '/contacts/v1/contact/vid/{}?hapikey={}'.format(vid, api_key)
h._call(subpath=None, url=subpath, method="DELETE")

[kostasgeo]

LGTM