gettext i18n

After getting a few advices and ideas from @nullstyle I started working on i18n of stellar-client using [`angular-gettext`](http://angular-gettext.rocketeer.be/). It's just the initial commit pushed to be sure I'm going into right direction. I will continue working after confirmation.

`angular-gettext` attributes and JS functions will allow to extract strings to `.pot` files which then will be translated using tools like [Transifex](https://www.transifex.com/). Extracting strings to `.pot` files will be automated using [`gulp-angular-gettext` plugin](https://github.com/gabegorelick/gulp-angular-gettext). When translations will be ready, they will be pulled from Transifex (ideally during `dist` gulp task). @nullstyle also shared a great idea to create a gulp plugin that statically translates templates where possible, reducing the amount of work each browser does. I developed a first version of this plugin and created NPM package: [`gulp-angular-gettext-export-html`](https://github.com/bartekn/gulp-angular-gettext-export-html). I checked how it works with a few templates from stellar-client and results are promising (`npm test`). There's still much work to be done though.

When it comes to `angular-gettext`, here are my thoughts after checking templates:

* Splitting conditional bindings like:
```html
   {{ showAddress ? 'Hide' : 'Show' }} your Stellar Address
   {{ claimRewards.isLoading() ? 'Loading...' : 'Claim now' }}
```
to following forms:
```html
   <span ng-if="showAddress">Hide your Stellar Address</span>
   <span ng-if="!showAddress">Show your Stellar Address</span>
```
will make translators less confused.
* It would be better to move URLs from texts with `<a>` links like:
```html
Please <a href="https://www.facebook.com/help/398085743567023/">verify</a> your Facebook account and try again.
```
to controllers or use [`ngInit`](https://docs.angularjs.org/api/ng/directive/ngInit) (rather bad idea).
* Multiline strings like:
```html
    Verify your email address to activate your password recovery feature.<br>
    Your recovery code will be sent to your email.
```
can be split into two or more separate strings.
* Uppercase strings:
```html
YOUR STELLAR ADDRESS
```
probably will pollute `.po` files. Maybe it's better to create a directive that will change node's text to uppercase?