Adds more documentation about flash and flashbox

Added documentation on how flash can be used in view or layouts,
explaining that any key can be used.
An example shows how to take advantage of that to use flash keys as css
selectors.
Finally, a small example of how flashbox is used in views is shown.
Hopefully, this fixes #64
Fixes typos in sessions.md

Fixes my own typos, introduced in previous commit.
Merge pull request #65 from leucos/fix-64

Adds more documentation about flash and flashbox