updated library to allow for passing in url

This does not follow the newly discussed standard: scottmotte/sendgrid-opensource-proposals@479627b as I made these code changes prior to it being posted.

Do you feel it's good to add this now (even though it may not be what we eventually determine) or keep the code change from occurring until the true standard is determined?

We can always point people to this PR if they need to change the URL at present.