modify labels to be clearer for fav pairs

Label favorite trades during the different scenarios.

- It should say "Favorite trades" when the form is reset/new
- When the currency pair is valid, it should prompt "save this pair?"
- I added a action "delete pair" that relates to #898 that shows up when a favorite pair is selected. But I can only do the template code, will need your help to fill in on the controller end. 
- I need another menu item that shows up when there's no saved pairs. Currently it shows an empty drop down without telling them what they should do. So I was thinking to just have a helper message that shows up in that empty dropdown that says "Select currencies to add a favorite pair" if there's no saved pairs. This helper message should also show up when pair selected has already been saved "This pair has already been saved" or if it is not valid "the pair you've selected is not valid".

Fixes #898.
Fixes #907.
One way is to enable tabbing is to use `ng-if` to show and hide the menu. When hidden, it doesn't appear in the browser's tab index. When shown, it does.