add Fiber#alive?
Merge pull request #1267 from skandhas/pr-add-Fiber-alive_p

add Fiber#alive?
Add test for raise on resume after fiber is dead
Test for raise when yield is called on root fiber
Merge pull request #1268 from carsonmcdonald/fiberextests

Add a couple Fiber tests
small refactoring over word boxing
Merge branch 'master' of github.com:mruby/mruby