#207 assert for Comparable

Draft to introduce Assertions.assertThat() for Comparable using natural ordering, cf. issue https://github.com/joel-costigliola/assertj-core/issues/207

Introduces a GenericComparableAssert (not sure about the name though) which is just a concrete subclass of the AbstractComparableAssert to keep the current abstract/concrete split for asserts.
I was not sure you would prefer collapsing the AbstractComparableAssert and GenericComparableAssert tests together so I duplicated the tests - but maybe we should collape them and test only the Generic one to help contain the number of tests.

Please feel free to suggest changes or tell me if something is wrong.