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.
Actually, there stands 'To apply these terms, attach the folowing notices to the library'. I don't think this is meant to be filled out right here but when added to a file!