Standalone asserts

Pull out all totem asserts into `totem.asserts`.

    > require 'totem'
    > t = torch.ones(5)
    > totem.asserts:assertTensorEq(t, t, 1e-5, "message"))


Also added some meta tests, so we can test the totem asserts (and by extension, totem itself) with totem :) 

`totem.asserts` is a Tester object with patched `_success` and `_failure` methods. It may be cleaner to expose its methods as functions rather than methods (by forming closures around the object) to replace the `:` with a `.` in the code above. Let me know your thoughts.