nest-simulator Refactoring the startup.

The function neststartup() in nest/neststartup.h/cpp is used both for starting up the sli-nest executable as well as the PyNEST module. A compile time flag _IS_PYNEST is given to compile this file to differentiate what is build. In master this is buried in pynest/Makefile.am, which uses the files in ../nest/ as sources.

In cmake (#213) a target nest_lib in nest/CMakeLists.txt is build. Depending on the availability of Python this is build with -D_IS_PYNEST and later used as a library to pynestkernel.

I propose, that we should refactor the method: only have NEST specific startup code inside it; then create some method sli_nest_startup(), which calls the neststartup() and initializes all sli relevant code and create some method pynest_nest_startup(), which calls the neststartup() and initializes all pytnest relevant code.