Actually initialise the unique_ptrs with live objects

... instead of nullptr.
Close the tox object after use.
Check that double close causes an exception.
Use pkg-config instead of custom libtoxcore finding script
Use JUnit4 expected exceptions instead of manual catch.

Also added a test for doInterval.
Merge pull request #6 from pippijn/master

Use pkg-config instead of custom libtoxcore finding script and add some tests