added ability to change uri via options

this brings the library in line with:
https://github.com/scottmotte/sendgrid-opensource-proposals/blob/master/proposals/PORT.md

as this uses parameters that also could be used by request, this may cause
problems with those using the protocol parameter
(however, I believe that any use of the protocol parameter in the past, would
not have worked, anyway)
Merge pull request #123 from sendgrid/url-changes

added ability to change uri via options