First version of the Thread interface

This commit includes the first version of the interface of a thread.
This interface is not completed but, when implemented, the thread should
be usable.
Interface of the ThreadError execption

The first version of the exception's interface was outlined.
Added copy contructors to errors

The error classes of the POSIX module did not have copy contructors. The
were added to allow the construction of the chain error implemented in
the base class.