RFC: Added doc/DocCheck.jl: functions for checking documentation status

The following are defined in doc/DocCheck.jl

* `undefined_exports(m::Module)`: list the undefined exports in a module
* `undocumented()`, `undocumented_by_file()`, `undocumented_rst()`: produce a list of undocumented methods in various formats
* `isdeprecated(fn)`: true if fn is deprecated

This request also:
* defines `current_module()`: return the current module
* defines `functionlocs(fn)`: return all function locations (`functionloc(fn)` just returns the first)
* commented out undefined functions in exports.jl (these could probably be removed)

Edit: Updated to reflect current contents