Supporting multiple screens

People often ask how Anvil supports multiple screens. Currently it only provides an `isPortrait` helper.
All "media queries" (in terms of CSS) end up with checking current `Configuration`, which can be retrieved from `Resources` assiciated with the current `Context`:

```
if (getContext().getResources().getConfiguration().orientation == Configuration.ORIENTATION_PORTRAIT) {
// portrait layout
} else {
// landscape layout
}
```

To keep Anvil simple we can remove `isPortrait` and make users write media queries manually. Or we can find a minimal subset of media queries to satisfy most needs.

Configuration allows you to detect:
- Orientation (landscape, portrait)
- Screen size category (small, normal, large, xlarge)
- Screen DPI
- Screen physical size in inches
- Screen geometry (square, normal, long, or specific aspect ratio)

From the above I find the most practical to detect orientation and to tell normal (phone) layouts from large (tablet) layouts. This means that `isPortrait` is here to stay, but the screen size could be checked with different functions:
- `isTablet()`, `isTv()`, `isPhone()`
- `isLarge()`, `isXLarge()`
- `displaySize()` that would returns size in inches, e.g. `if (displaySize() > 6.5) { tablet(); } else { phone(); }`

etc, etc. What are your thoughts?

P.S. Anko supports lots of configuration media queries - https://github.com/Kotlin/anko/blob/master/doc/ADVANCED.md#configuration-qualifiers while Scaloid supports only a few - https://github.com/pocorall/scaloid/blob/master/scaloid-common/src/main/st/org/scaloid/util/Configuration.scala