Merge pull request #3 from michalt/cleanup

merged Michal's cleanup of semilinear set implementation
- replaced Matrix implementation by old one (strange segfault when multiplying matrices)
- tried to implement sl-set parser (not successful yet)
- test-switch in main() for adhoc testing
added implementation of prefix semiring
slset-parser now working
new test-grammar... binary trees.
update README and improve cmake configuration

Write down some basic notes about the current state of the
project in README and make it possible to specify alternative
locations of Boost and Cppunit.