Make tests better

- [x] Make tests run in a browser
- [x] Integrate with [sauselabs](https://saucelabs.com)
- [x] Make browser tests run in travis-ci (connect to sauselabs)
- [ ] Build mock server or find some other solution that works in a browser and in Node
- [ ] Rewrite tests in `test/wallet.js` to be able to run them individually and without running stellar-wallet server (#7)

**Ongoing development**