[MRG] Do not introspect arbitrary objects by default

[sdvillal]

Tackles #9.

Also highlighted the need for:

• Cycles in the object graph should be handled to avoid giagantic stacktraces. A look at how pytest does it (probably capturing the relevant exception and simplifying the message) might be useful.
• We need a proper way to create plugins contexts (although that is hard and dangerous given the library philosophy).
• The need for proper documentation (in this case what whatami will do when finding an arbitrary, non-whatable object).