Recovey middleware : improve stacktrace

debug.Stack is not only deprecated (scheduled for removal), but also broken for package path containing dots (like "github.com/codegangsta/martini"...).
We fix this by introducing a private copy of debug.Stack that handles this (and also removes 1 more level of stackframe for cosmetic reasons).
Merge pull request #161 from vanackere/recovery-stacktrace

Recovey middleware : improve stacktrace