scripts to batch compose products

i have added some scripts to compose products.

I use them for regression testing e.g. i compose products using a known, good version of fstcomp, i edit the composer and compose again; then, i use a diff-tool to check if the composed products are still ok.

`compose_examples.py` composes some of the fstcomp examples using default composition parameters.
`compose_regrtest.py` also composes those examples, but also builds some products using variability encoding - you will need to fetch those product lines beforehand by running `introduce_varenc_examples.sh` (also included).

I did not know where to put these scripts exactly, so i created a `scripts` folder inside fstcomp; fstmerge also contains such a folder.

Right now `compose_regrtest.py` causes some composition errors - this might be because i have selected illegal products or maybe because of bugs in the composer - i will investigate that.

Also, `compose_regrtest.py` does not currently use all of fstcomp's supported languages and command line options - it's a start.