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.
Integrated unit tests into the build system

Availability of "check" library will be automatically tested by
configure, if the library is not available tests will not be build.

Further, an option to manually disable/enable compiling of unit tests is
provided: --enable-tests / --disable-tests

To compile and run the tests use:

make check
Integrated new files into the build system
Fixed missing include headers
Make sure m4 directory is in the git repo

...otherwise autoreconf -i will fail.