Configurable endianness of data

Working on https://code.google.com/p/scanmem/issues/detail?id=70

This patch provides a runtime option, `endianness`. possible values:

0: data has same endianness as host
1: data is little endian
2: data is big endian

Commands that operate on words (set, write, and comparisons) respect
this setting when reading and writing.

Nothing is changed in the GUI, this is a command-line only option, for
now.

What do you think?