Fix is_exact() by changing condition precendence and add tests

matchnot in is_exact was wrongly XORed with the rest of
conditions. is_exact() is supposed to check whether a needle is found
in haystack for the given substring position and XOR that value with
matchnot.

The conditions preceeding XOR matchnot were not grouped
together previously. This resulted in the XOR operation getting
executed before all conditions before it were evaluated.
Implement basic parser and updated tests for it. Fix #2

Parser handles single character, multi-character (words and
sentences), punctuations and, above all, unicode properly
Add Acknowledgements to README.rst