Make Makefile more well-formatted
Add UNREACHABLE helper macro
Add sesh::async::Result class template
Make Variant class template non-final
Add Variant constructor to initialize from function
Add Delay class template
Add DelayHolder class
Add Promise class template
Add DirectInitialize class
Add SharedFunction class template
Add Future class template
Rewrite Future implementation for clang 3.3

Clang 3.3 fails to treat function-local classes correctly.
Merge pull request #71 from magicant/feature/async

Implementation of asynchronous return value