Rename events.h -> mbed_events.h to avoid potential name collisions

[geky]

Note from @pan-:

Headers in this library should be prefixed otherwise they can easily clash with over headers. For instance events.h is too generic and can be taken by other libraries.

The events.h name may be a common name for header files. On faulty build systems, there is a potential for this common name to cause the wrong header file to be included.

discussion https://github.com/ARMmbed/mbed-events/issues/6 cc @pan-, @0xc0170 fixes #6

[0xc0170]

+1