Xmalloc: Fail if size == 0
Merge pull request #34 from antoinealb/malloc-fail-zero

Xmalloc: Fail if size == 0
Changed mutex API to static allocation.
Added mutex implementation for uCOS-III.
Changed semaphore API to static allocation.
Consistent comment style.
Added semaphore implementation for uCOS-III.
Rename files to singular "semaphore".
Added missing include and fix for compiler warning.
Added missing include & fix for compiler warning
Implement static allocation of mock mutexes
Fix documentation
Added threading header
Added threading implementation for uCOS-III
Updated README with threading example.
Rename os_thread_sleep_ms()
Updated package file.
Added mutex source to package file.
Added semaphore source to package file.
Updated documentation.