Introduce basic memory types

The files 'src\com\binaryphysics\appleseed\Byte.java' and
'src\com\binaryphysics\appleseed\Word.java' contain the classes that represent a
byte and a word. These objects are likely to be stored in a large array to make
up a block of memory.

While the memory of a MIPS implementation is stored in Words MIPS is byte
addressed and thus there needs to be a way of getting hold of bytes.

When a Word is constructed from a group of Bytes the assumption is that the
architecture is little-endian.