Activefilt

Dear friends!

I implemented new tool for qucs. It is QucsActiveFilter. This tool allows to design active Butterworth, Chebyshev, Cauer, Bessel and User defined active filters. Sallen-Key, Multifeedback and Cauer topologies are implemented. I hope, it will be usefull.

Added new subdirectory qucs/qucsactivefilter, that contains all of my new sourcecode.

If you will merge this pull request, add directory entry qucs/qucsactivefilter to main CMakeLists, please.

![qucsactivefilter](https://cloud.githubusercontent.com/assets/4920080/3537940/2f1dee7c-0828-11e4-83cf-06319b6c4655.png)

![qucsactivefilter1](https://cloud.githubusercontent.com/assets/4920080/3538155/845b50ac-082b-11e4-849c-422c6839b91c.png)