Added methods to handle threads

The first draft of the Thread class was implemented. Threads can now be
created and started from a Runnable object.
Next steps are improve error handling and start threads using
subclasses.
Added methods to handle threads

Threads can now be started from subclasses or from Runnable objects.
Next steps are error handling and adding Doxygen documentation to code.
Documentation added

Some Doxygen documentation on thread class was added.
Added the wait-for-a-thread feature

The join() method was added to the thread class in order to allow
waiting for a thread termination (also to simplyfy tests!).
Documentation added

Some Doxygen documentation was added to thread class.
Added an error class

An error class was added to represent with an exception an error
occurred during a POSIX Thread call.
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.
Completed the error management for threads

The required exceptions for error reporting in threading classes were
implemented. The next step is to include those exception in the Thread
class.
Error management added to thread

THe new error managemente mechanism was added to The Thread class but...

  IT HAS NOT BEEN TEST-DRIVEN!

I need to continue but I promise to figure out how to try this kind of
things.
Implemented the exit function for threads
Added the Process class

Al the files that will handle the multiprocess classes were added.
Testing the implementation seems a bit difficult because after launching
test SUT, it is running in the context of another process and there is
no shared memory. I leave this class here until I figure out how to test
this feature.
Added the first test for process execution

The code needed to execute processes from runnable objects was added.
The next effort will go on make processes executable from subclasses,
refactor the code  and write some Doxygen documentation.
Documentation added

Some Doxygen documentation was added tho the Process class.
Test refactored
Test implemented

The code to execute a process from a subclass was implemented.
Documentation added

Some piezes of Doxygen documentation were added to the interface of the
Process class
Method implemented

The Process::exit() method was implemented using an existing test
New class

Added the first version of the interface of the ProcessError exception
class