View source for Sx assembler

Work is in progress on a [http://boost.org/libs/spirit/doc/introduction.html boost.spirit] based SX assembler.

The sx assembler consists of 2 parts:
* frontend; create a syntax tree out of the program text. There is a working spirit grammar without actions (the actual tree building). Open items are:
** include
** macros
** recognizing opcodes that start in the label column (the first column). Currently, a word that starts in the first column will be parsed as a label, never as an opcode.
* backend; create a list- and hex-file out of the generated parse tree.