Memory manager

So I merged picotcp again with the memory manager and i've run some UDP tests that we used before. I do believe that further testing is still needed before it is stable enough to be used in a project, but this can be done by enabling the memory manager in existing projects and then check if everything still works. 

NOTE: it is not enough to only enable the compiler flag MEMORY_MANAGER=1!!! One must also call for example pico_mem_init(10*4096) to let the memory manager know it can use 40k of RAM and to initialize everything!!

Another remark i have to make is that i'm still unable to run picotcp with the mem manager enabled on my AVR board. I know where it goes wrong, but I can't figure out why since I dont have decent debuggint tools for my platform...

As before the unit tests can be compiled by doing make units_mm MEMORY_MANAGER=1 and then can be executed by doing ./build/test/units_mm

Grtz,
Sam.