Require guard ~> 2.8
Inheirit from Guard::Plugin now

See https://github.com/guard/guard/wiki/Upgrading-to-Guard-2.0#changes-in-guardguard
Update Guard::Puma's initialize method args
Add note on Guard 2.8 dep
Add note about initialize update
Bump rake dep to 10.2+
Fix version formatting
Prepare for 0.3.0 release
Release 0.3.0