Removed BSD license in favor of 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.