Add support-v4 views and widgets

Changes:
- Added support-v4 views and widgets
- Added implicit context to `SFragment` to keep API the same as `SActivity`
- Update to SBT 0.13.1

#### Attention needed

Implicit context in `SFragment` is potentially harmful because it will be `null` if the fragment is not attached to an activity at that point. I also added a comment explaining this but it is probably not enough.

It might be better to remove it and let users add it by themselves explicitly when it's needed. What do yo think?