Removed BSD license in favor LGPLv2.1

As discussed in #43 , this PR changes the license from BSD to LGPLv2.1.

At the moment, @neithernut and @manuelmessner are the only contributors (besides me) to this library. Both of them should add their acknowledgement of this PR here.

By adding this PR, they accept that all code they have contributed so far is now licensed under LGPLv2.1.
The last few paragraphs are not part of the license but a note on how to use the license. I do not remember reading any such thing while scrolling down LICENSE files in either repositories or tarballs. If you dig up any library of program, it will most certainly _not_ contain the last section.