Have nupic.core build output a shared library artifact instead of the static lib

[vitaly-krugl]

Currently, nupic_core_solo is a static library and gets repackaged with the external dependencies into a combined static library that the binaries and Python extensions link to.

This change is to:

• Remove the combined nupic_core static library
• Change nupic_core_solo to a shared library and include the external dependencies for linking
• Change the extensions to dynamically link to the nupic_core shared library
• It looks like the logical place to put the Python-specific files (called {{src_py_support_files}} in {{src/CMakeLists.txt}}) is in nupic_core_solo but it may be better to leave them out and include them when compiled the Python extensions since they aren't really part of the C++ API that we want to expose in the shared library. But whoever is looking into this can make a judgement call.