Add MutexAttr

MutexAttr is a simple wrapper to provide mutex lock support (via
"monitor" stdlib) on a per-attribute basis in an object.  Methods
performed against the contained field are invoked within the lock's
#synchronize() method, thus ensuring that concurrent access to the
attribute does not result in a conflict.
Convert to extension

Due to transparency issues in an object-based wrapper, an extension has
been written that hacks Module and introduces the functionality at a
class-level without altering the values of the attributes themselves.

Use the new .mutex() method in any module or class and pass the desired
attribute symbols (no @ prefixing at this time) to wrap them.  The
extension will optionally define and create a reader method if the
analogous instance variable does not yet exist.
Extension bugfix

Corrects error in instance variable handling for the mutex extension.
Merge pull request #10 from florf/mutex

Confirmed.