Converted asserts to errors, plus a few other changes.

- Added several new error constants.
- Changed return type of Thread::setPriority(), Semaphore::put(), and
  Mutex::put() to status_t.
- Renamed SemaphoreHolder to LockHolder.
- Removed inline keywords in ar_kernel.h.
- Removed definitions of AR_MAX_NAME_LENGTH.
- Changed NamedObject to have a const pointer to the object name rather
  than internal storage for a copy of the name.
- Converted asserts to condition checks and error code returns in
  Mutex::put(), Semaphore::get(), Thread::init(), and Thread::setPriority().
- Made Thread::setPriority() not do anything if the priority is the same.
Added basic semaphore test case class.
Moved documentation comments for public methods to ar_kernel.h.

- Also rewrapped all documentation comments to 100 char width.