Make tests better

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