RFC: Added undocumented(), undefined(), isdeprecated(); doc and export updates. 

* `undefined(m::Module)`: list the undefined methods in a module
* `undocumented()`, `undocumented_by_file()`, `undocumented_rst()`: produce a list of undocumented methods in various formats
* `current_module()`: return the current module (is this always Main?)
* `isdeprecated(fn)`: true if fn is deprecated
* commented out undefined functions in exports.jl (these could probably be removed)
* documentation fixes (in manual) and update

Shouldn't be very controversial, but let me know if, e.g., you don't want all three `undocumented*()` functions.