Changed trading form label to [Buy/Sell] [currency]

This implements https://github.com/stellar/stellar-client/issues/850

Initially, tradeOperation was all lowercase and the button would become `[buy STR]`. I needed a way to make it uppercase. There were several ways:

1. Using css uppercase (used here: https://github.com/stellar/stellar-client/blob/trading/app/templates/trading-form.html#L44 ) (may interfere with i18n/l10n due to edge cases in some languages (see below))
2. Creating an AngularJS filter for update (Might adversely affect i18n/l10n due to edge cases)
3. Detecting if tradeOperation was 'buy' or 'sell' and then outputting the correct string (best for i18n/l10n but clunky)
4. Changing tradeOperation to be represented in the correct casing and directly using it  (simple and good for i18n/l10n)

There are edge cases in capitalizing and not all the browsers do css capitalization the same. Read more about the edge cases here (also take a look at browser [in]compatibility): https://developer.mozilla.org/en-US/docs/Web/CSS/text-transform

I'm hoping to avoid problems with capitalization in the future (as Drupal had): https://www.drupal.org/node/1496418

Because of this, I think that it would be best to let translators decide on the capitalization and not interfere (with css or a filter).
I would revert to have lowercase in JS scripts. Then in HTML files we could do something like:
```html
<span ng-if="formData.tradeOperation === 'sell'">
   Sell
</span>
<span ng-if="formData.tradeOperation === 'buy'">
   Buy
</span>
```
It will solve all issues when we'll be translating stuff.