Added many more API endpoints...

Filters, invalid emails, mail and credentials.

API methods now return a Sendgrid::Web::Response which correctly
parses the response body using Oj.

Faraday was switched out as the test stubs were not working as
required. Now HTTParty is used and the internal connection method
points to a private "API" class that includes HTTParty.

A shared spec example was created to make testing endpoints easier
as well as reducing the amount of duplication when stubbing the
connection.
Big documentation update using yard.

Fixed a regression caused by switching to HTTParty which meant
parameters weren't being included in query string. I have updated
the specs and turned `default_params` into a method that accepts a
hash. The provided hash is then merged nto the query string as is
expected. All previous merges are now using the method.

On a side-note I have began trying to enforce a 70 character width
in-order to help keep the code style consistent.
Updated the README
Updated specs to better check provided parameters

Version bump

Moved version to autoload in-line with other classes/modules.

Ensured that requiring `sendgrid-web` correctly bootstraps the gem.