Change License to LGPLv2.1

[Citrullin]

Under GPL, everything linked against it has to be licensed under GPL as well.
Therefore, it is not suitable for business environments. On the other side combining GPL with MIT/Apache is not possible, since it would become GPL. I recommend to use LGPLv2.1, since you can link against it, without publishing the source code and combine it with other licenses, such as GPL.