Add trailing slash to urls to avoid redirects

[esonderegger]

I don't know if this is the result of a recent change to api.open.fec.gov or if it has always been that way, but currently requests to urls like https://api.open.fec.gov/v1/candidates get redirected to urls like https://api.open.fec.gov/v1/candidates/ before they return a response. The requests library takes care of this for us by following redirects. Unfortunately, both the original request and the redirected request count against the user's API rate limit. That means users of the pyopenfec library can only make 500 meaningful requests per hour with the FEC's default 1000 request/hour rate limit.

This PR addresses the issue by adding a trailing slash when the url is created. This could potentially cause problems if either a resource has a trailing slash in it or if there is a resource on api.open.fec.gov that does not expect a trailing slash. However, currently there are instances of resources in the pyopenfec code base that have a trailing slash and all the resources listed at https://api.open.fec.gov/developers/ contain a trailing slash.