consistent function naming

The names of functions/macros in `monads.core` namespace are not so consistent, some starts with a 'm', like `mdo`, `mfail`, some does not, like `tell`, `asks`. What if remove all the 'm' prepend, encourage user to do so:

``` clojure
(require '[monads.core :as m])
(m/do ....)
(m/fail ....)
(m/tell ....)
```
