Update Mocha and NYC test configuration (#102)

- Add TSNode to get proper coverage on TS files
- Tell Mocha to look for tests in subdirectories
- Configure NYC to look for coverage on all src files
- Move `source-map-support` to being a dev dependency
- Ensure NYC keeps us at 100% code coverage

packages/ripple-keypairs/.mocharc.js

@@ -3,6 +3,13 @@ module.exports = {
   slow: 500,
   timeout: 5000,
 
+  // Required to get proper coverage on TypeScript files
+  // transpile-only is required if we use custom types
+  require: ['ts-node/register/transpile-only', 'source-map-support/register'],
+
+  // Look for tests in subdirectories
+  recursive: true,
+
   // Check for global variable leaks
-  "check-leaks": true,
+  'check-leaks': true,
 }