Projection API simplifications

This PR is opened for discussion. It simplifies the use of projections in the common case of a map using EPSG:3857 but the user wanting to think in EPSG:4326.

In short, it adds two convenience functions, `ol.toMap` and `ol.toUser`, that convert between the map and user projections:
```javascript
ol.toMap([latitude, longitude]); // returns [x, y] in EPSG:3857
ol.toUser([x, y]); // returns [latitude, longitude] in EPSG:4326
```

This makes the API easier to use, without the complexity of juggling two different projections internally. Note that coordinate order is respected, i.e. the user works in the order latitude, longitude. See the examples for how it is used.