Better integer test on ModelStorage instantiation
The test done at [1] is not really reliable because some other values than integer could pass it like a string with an integer or a float. Of course it will fail after but it will be more obscure issue see [2].
[1] http://hg.tryton.org/trytond/file/9594dc72ebe3/trytond/model/modelstorage.py#l1173
[2] https://groups.google.com/d/topic/tryton/4EB9PB2-hzM/discussion