Generate API docs for some missing sub-modules in astropy.utils

This PR adds some `automodapi` directives for `astropy.utils`.

I noticed in https://github.com/astropy/astropy/pull/2857#issuecomment-52617740 that e.g. `astropy.utils.lazyproperty` was missing from the online docs:
http://astropy.readthedocs.org/en/latest/search.html?q=lazyproperty

One issue with this approach is that with these separate `automodapi` directives, e.g. the `lazyproperty` function shows up at `astropy.utils.decorators.lazyproperty` instead of at `astropy.utils.lazyproperty` which is location users should import from.
Is this acceptable or if not, how exactly should it be structured instead (automodapi for `astropy.utils` or manual listing to get nicely sorted functions by topic)?

cc @embray @astrofrog