Added gitignore exception for the unittest folder.

the "unittest" line in the gitignore was also ignoring the
"test/unittest" folder with all of the unit tests in them.

.gitignore

@@ -283,6 +283,7 @@ paket-files/
 *.hex
 gravity
 unittest
+!test/unittest
 
 # Debug files
 *.dSYM/