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.