Fixes for issues #6 and #7

[TomWeishaar]

Please ensure the following before submitting a PR:

• if suggesting code changes or improvements, open an issue first
  Issues 6 and 7.
• for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
• for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
• if changing documentation, edit files in /R not /man and run devtools::document() to update documentation
• add code or new test files to /tests for any new functionality or bug fix
• make sure R CMD check runs without error before submitting the PR

The replyto issue, #6, is fixed in sendemail.R at lines 108-111.

The other changes in the two files have to do with issue 7. I fix this issue by allowing the user to enter key, secret, region, and force_credentials = TRUE in the send_email() function. The region in this case is the region of the AWS SES server, which, in the long run, is unlikely to be the region of the user's instance. I also added a bit to the Details about how to get the needed key and secret from AWS.

The fix also involves putting an if() statement around the locate_credentials call in http.R so that the user's credentials aren't over-written when force_credentials = TRUE and so that the region parameter in send_email() remains the SES region, not the instance region.

As an added side-effect, with these changes aws.ses allows a user with the proper credentials to send email from any computer, not just an instance on AWS.