add TypedPreferences
add extra; access point to Activity Intent.Extra
Merge pull request #92 from wyverny/master

add extra; access point to Activity Intent.Extra
fix a layout bug on example
Merge branch 'master' of github.com:pocorall/scaloid
update README.md
Improve on intent.extra support using monad ops and use a value class.

Switching to Option.flatMap avoids all the checks for Some/None.
 The value class avoids allocation overhead of Extra objects through desugaring, since there's no member data.
 While here, implement the rest of the extra data types.