feat(navBar): add sensible defaults

BREAKING CHANGE: Before, if ionNavBar back-button-type attribute was not defined,
no back button would appear. Now the back button defaults to an os-specific
icon, with the previous view's title.

Additionally, before, if ionNavBar animation attribute was not defined,
no title animation would happen.  Now the ionNavBar animation defaults
to nav-title-slide-ios7.

To migrate, change your code from this (used to be no back button, no
title animation):

```html
<ion-nav-bar></ion-nav-bar>
```

To this (no back button, no title animation):

```html
<ion-nav-bar back-button-type="" animation=""></ion-nav-bar>
```