updated sendgrid obj to be similar to other libs

- changed username/password to api_user/api_key
- allowed api_user, api_key, url, and version to be accessed and changed from the sendgrid object

this makes the sendgrid object to the sendgrid object in our node and python libraries
updated lib to allow for setting url

does not follow newly discussed standard
https://github.com/scottmotte/sendgrid-opensource-proposals/commit/479627b0e912fbb5315d3e4d11c3a2ad2e8cef77
Merge pull request #96 from sendgrid/add-url

updated library to allow for passing in url