Rename core directory because of autoconf name clash

While doing the checks configure might generate "core" files and will
then try to remove them. Having a "core" directory generates an error
while runing the configure script.

There's no workaround but to rename the core directory.
Implemented autotools based build scripts

supported options:

--with-dependency-search=DIR    will tell configure to look for various
dependencies in DIR/include and DIR/lib

Alternatively you can also specify libsodium header and libs location
with --with-libsodium-headers and --with-libsodium-libs if it is
installed elsewhere.

Ncurses and libconfig are handled via the default pkg-config way, see
./configure --help=short for detailed information.

The tox library is compiled as libtoxcore in shared and static variants,
public headers are installed to ${prefix}/include/tox

A pkg-config libtoxcore.pc configuration file is provided.

Use ./configure --help for a full list of configure options or
./configure --help=short for the options that I added.

To generate the configure script after pulling from git use:
autoreconf -i

To generate a release tarball use:
make dist

Unit tests are handled by the libcheck library integration that is provided
by autotools, use:
make check

to compile and run the tests.

Unit tests are currently optional, i.e. - if the check library is not
found on the system, then tests will be disabled.

The cmake build still works, so it's possible to build the project either
with cmake or with autotools, depending on ones preference.
Fixed missing includes