Updates for Stata 15

[wbuchanan]

• Need to add entries described in a Statalist post by Dave Airey.
• Need to add test of Stata version to brewtheme and brewscheme.
• Need to add method to handle alpha layer transparency for cases where end user is using Stata 15 or later.

[wbuchanan]

See Issue #58 for new issue to address all of these issues.