add method to check if user exists

[MiriamSexton]

part of #5

pull from https://github.com/industrydive/sailthru_tools/blob/master/hosted_pages/tests/sailthru_user_factory.py#L16

[elidickinson]

I struggle a little with how much abstraction we want to insert between the interface this library provides and what actually needs to happen with the API. Adding a user_exists function would be ideal for consumers of this library, but if they use it to write code like:

if client.user_exists(email):
    data = client.get_user(email)

Then they've actually called the get_user API twice in a row. Would it be better to just have get_user throw an easily catchable Exception class in the event a user doesn't exist?