Build frees input string memory

[usamec]

This line should not be here:
https://github.com/mpetri/FM-Index/blob/master/FM.cpp#L127

I believe, that in C++ most people believe in RAII principle and do not expect FM index to free input string memory.

[mpetri]

you are right. I'm not really maintaining this anymore. You should probably use the sdsl-lite library (https://github.com/simongog/sdsl-lite) which contains many more succinct text indexes besides the FM-index and uses proper c++11 coding practices (at least for the most part).

[RagnarGrootKoerkamp]

@mpetri could you add the fact that sdsl-lite should be used instead to the top of the readme (or even archive the repository)?

As a user there's already too many fm-index implementations to choose from, and without context on the status of the various codebases it's hard to choose. Having one less in the mix helps :)