further-generalize SendGrid.java

[owades]

Kay's note on #98:

We are actually pretty close to making this class a very light-weight java API for sendgrid. It's kind of a separate project, but really what's missing is:

• changing name of map back to blobMap
• pass in a list of mime types instead of assuming image\png
• remove hardcoded sender, and instead use SendGrid.Builder approach to create and configure instances