Gulp task to run protractor tests

Gulp task to start `protractor` tests. Before running tests it ensures webdriver is installed (`ensure_webdriver_standalone`) and starts it (`webdriver_standalone`).

Usage:
```
gulp test
```
It's not run indefinitely. We [kill it](https://github.com/stellar/stellar-client/pull/867/files#diff-b9e12334e9eafd8341a6107dd98510c9R343) after protractor ends it's job.