Add flow-bin to dev dependencies and ignore it in flowconfig

Add flow to api/server/server.js

Add flow check to src/api: index, accountinfo, balances, orderbook..

orders, settings, transaction, transactions

Add flow typecheck to api/ledger/pathfind

Use eslint plugin flowtype to allow flow type annotations

Babel-eslint emits errors when using the type keyword. This plugin works
around the issue by stripping flow annotations before linting.

Source: https://www.npmjs.com/package/eslint-plugin-flowtype

Add flow to ledger/parse/account-order, trustline, orderbook-order

src/api/ledger/orders.js

@@ -1,3 +1,4 @@
+/* @flow */
 'use strict';
 const _ = require('lodash');
 const utils = require('./utils');