Implement memory protection

[alexrp]

Really simple stuff. Just need to respect the MEMPROTECT register and update the MEMSTATUS register. This should be done in the Memory class.

We do need to figure out what MEMRESERVE and MEMCONFIG are, though: http://forums.parallella.org/viewtopic.php?f=8&t=933

[alexrp]

OK, the latter two registers are implementation-specific and/or don't actually exist. We should ignore them.