Add support for eldoc-mode.

eldoc-mode displays docstrings in minibuffer for symbols and function
calls under point. Implemented by using existing completion functions,
which already return documentation and types.

Notes:

I use irony-mode in C++, and the way I get eldoc right now is with
c-eldoc. But irony-mode already has all the information it needs to
provide eldoc support, since docstrings (?) and especially types are
supplied by the auto-completion functions. irony-mode also, I think,
has the better support for compilation options, and also benefits from
not reparsing files as much.

So since the feature comes almost for free, I think it should be added
even though it might duplicate some other packages.

It works fine, and does not crash on the things that I've tried. Some
things are not implemented, such as displaying types for overloaded
functions where types of arguments matter.

I enabled this by default in irony-mode.

Thanks for irony-mode.

- Kirill