Add pkg-config file

Generate and install a toxcore.pc file to be used with pkg-config.
Merge pull request #519 from manuel-arguelles/pcfile

Add pkg-config file
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. Same goes for nTox and
DHT bootstrap daemon - they will be enabled or disabled depending on the
availability of ncurses (for nTox) or libconfig (for DHT bootstrap
daemon).

The above can be also tuned by:
--enable-tests / --disable-tests
--enable-ntox / --disable-ntox
--enable-dht-bootstrap-daemon / --disable-dht-bootstrap-daemon
Move travis build to autotools
Remove cmake build system