Various minor improvements

Hi! Here's a collection of changes which I believe make auto-complete-clang-objc more improvement. Thanks for creating this useful library.


196e595 titled "Enable omni completion for Objective-C message" is explained below.

Consider these code, with point at the position indicated by the caret:

    1. [];
        ^
    2. [self.window ];
                    ^
    3. [[NSBundle mainBundle];
                            ^

All of the above, invoking `auto-complete` will enable completion.


Cheers,

-Yasuyuki