Add unit tests.

[jdalton]

• Add test for specifiers that shadow those on the Object.prototype (Use Object.create(null) in a few more places. #78)
• Add test for .mjs (Add .mjs support. #108)
• Add test for exporting the same identifier more than once (Let first getter win if one already exists. #106)
• Add test for passing options to reify/node (Allow setting compiler options in node/index.js. #95)
• Add test for importSync with a key param
• Add test for modified ast
• Add test for ast compiler option