Make modules load lazily (#41)

* make modules load lazily

* fix lint errors

index.js

@@ -1,14 +1,14 @@
 const { BN } = require('./src/setup');
 
 module.exports = {
-  balance: require('./src/balance'),
   BN,
-  constants: require('./src/constants'),
-  ether: require('./src/ether'),
-  expectEvent: require('./src/expectEvent'),
-  makeInterfaceId: require('./src/makeInterfaceId'),
-  send: require('./src/send'),
-  shouldFail: require('./src/shouldFail'),
-  singletons: require('./src/singletons'),
-  time: require('./src/time'),
+  get balance () { return require('./src/balance'); },
+  get constants () { return require('./src/constants'); },
+  get ether () { return require('./src/ether'); },
+  get expectEvent () { return require('./src/expectEvent'); },
+  get makeInterfaceId () { return require('./src/makeInterfaceId'); },
+  get send () { return require('./src/send'); },
+  get shouldFail () { return require('./src/shouldFail'); },
+  get singletons () { return require('./src/singletons'); },
+  get time () { return require('./src/time'); },
 };