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/nyc.config.js

@@ -0,0 +1,19 @@
+module.exports = {
+  extension: ['.js', '.ts'],
+
+  exclude: [
+    '**/*.d.ts',
+    '*.js',
+    'test/**/*',
+  ],
+
+  // Assert we remain at 100% code coverage
+  'check-coverage': true,
+  'branches': 100,
+  'lines': 100,
+  'functions': 100,
+  'statements': 100,
+
+  // Required to get coverage reported on every file, including those that aren't tested
+  all: true,
+}