addition of append/3 and next_nth/3 ...

Hi Knut,

I added a few functions as discussed.
Also, there might be a problem with O(n) complexity with alternating 'insert' and 'find' operations, added a test to demonstrate it. It might be related to binary copying as described in http://www.erlang.org/doc/efficiency_guide/binaryhandling.html under "Circumstances that force copying".
Maybe you have an idea how to get around this.

Cheers,
Florian