stripe: support the limit parameter for Plans.get()
stripe: fix resources due to Stripe's new API

Now a Stripe's customer can own many subscritions and the count field has been replace by total_count.
segmentio: add batch import endpoint
Ducksboard password should be optional
Merge pull request #71 from DZPM/master

ducksboard: use empty string as the default for password

Using None makes the BasicAuth filter throw errors.
tests: add a test for using a Ducksboard service with no password
intercom: add support for Intercom's new API

The new API is a superset of v1 API. I've changed the endpoint and added the required 'Accept' header. I've added the new Count resource too.
Merge pull request #72 from javimb/intercom-new-api

intercom: add support for Intercom's new API
Merge pull request #70 from javimb/stripe-fix

stripe: fix resources due to Stripe's new API
zendesk: document how to use token-based authentication

Fixes #52.
auth: support making OAuth requests without a token

Requests to the Temporary Credentials Request endpoint as detailed in RFC5849
section 2.1 are made without the oauth_token paramter.

Make the OAuth filter Support getting a None for the token parameters and still
generate correct siguatures.