Added doc/DocCheck.jl: functions for checking documentation

* undefined_exports(m::Module): list the undefined exportss 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

Additional functions in base:
* current_module(): return the current module
* functionlocs(fn): return all function locations; (functionloc(fn) returns just the first)

Also:
* commented out undefined functions in exports.jl (these could probably be removed)
Merge pull request #3120 from kmsquire/undoc

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