Refactor YAML session with SessionFactory::File role

We shouldn't make every file-based session engine repeat the same file
handling code.  With the new Dancer::Core::Role::SessionFactory::File
role, they only need to provide a suffix, and implement serialization.
Add 'session_duration' attribute to SessionFactory

Session backends may need to limit the validity of sessions
regardless of cookie duration.  Rather than let every SessionFactory
add their own attributes, we should standardize it.
Moves session cookie setting from App to SessionFactory

The App now calls the SessionFactory to request that the cookie be set
instead of setting it directly into the Response.

This allows SessionFactories that need complete control of the cookie
(such as Cookie or PSGI) to override the default behavior.
prepare for tidyall
tidy everything
Fix inconsistency in Perlbal deployment example.
Thanks to Slaven Rezić for spotting this.
Merge pull request #163 from dagolden/topic/refactor-session-cookie-generation

Moves session cookie setting from App to SessionFactory
Merge pull request #162 from dagolden/topic/session-duration

Add 'session_duration' attribute to SessionFactory
Merge pull request #161 from dagolden/topic/session-file-role

Refactor YAML session with SessionFactory::File role
Merge pull request #165 from dagolden/topic/tidyall

Tidy all code files