Don't load mail gem until it's really needed

Implementing ruby autoload on big ugly mikel/mail gem:
huge improvement (around 20%) of padrino framework load speed
Evade autoload issues on limp ruby engines

Autoload is safe enough only on MRI 1.9, so disable it for other engines
Fix ruby 1.8.7
Merge pull request #998 from ujifgc/master

Improvement of Padrino's load speed