Devel

Changed minor things 
- added `using DataArrays` to remove the error message "unknown method isna()" when calling `describe()`, not sure what happened, but the problem has disappeared
- added a `run_tests.jl` script to launch all tests
- updated `REQUIRE` to tell that Julia >= 0.2 is necessary
- added a `show()` method for the `LikelihoodModel` type, to print something less scary than the default dump