Update TSConfig to produce type definitions (#79)

* Update TSConfig to produce type definitions

Also, add some stricter type-checking, and remove settings that were set
to the same value as the default for that setting.

I also reorganized the sections into the sections that TypeScript itself
generates when you boot up a `tsconfig.json` using their CLI.

* Downgrade target to ES6 for Node 8 support

* Add engines specification to package.json

Make it clear that we support Node 8 and above.

packages/ripple-keypairs/package.json

@@ -48,5 +48,8 @@
     "type": "git",
     "url": "git://github.com/ripple/ripple-keypairs.git"
   },
-  "license": "ISC"
+  "license": "ISC",
+  "engines": {
+    "node": ">= 8"
+  }
 }