Check whether mem file is open in Memory::IsValid

[dkrutsko]

It might be a good idea to check whether the /proc/mem file is actually open as part of the Memory::IsValid function.

[dkrutsko]

So even though the memory file is being checked by the actual reading and writing routines, there should still be a way to determine whether memory could be read or written to, even when other functionality could still work. Since #33 is still under consideration, there might be some set of functions in the Process class to check for various supported features. One of them could be memory manipulation routines.