Compile and install tox core as a shared library

Right now only Messenger.h and network.h are installed as public
headers, I am not sure what the public API is but so far the toxic and
nTox clients seem to use onle these two.

This surely needs to be reviewed in the future. Also, CLIENT_ID_SIZE
define and the extern uint8_t self_public_key variable should probably
go to some common header.