Improves Ctags Executable Detection.

Now runs 'ctags --version' the first time and checks if it is actually
CTags. On Mac OS X, `ctags` is linked to an unrelated program that is
not actually `Exuberant Ctags`.

Should resolve #17.
Bumps version to 0.1.18.
Updates changelog for 0.1.18.
Build 0.1.18
Merges branch 'release/0.1.18' into develop
Improves CTags generation messaging.

* If Ctags is not found, no generation messages.
* :Wctags! shows an error message if Ctags was not found.
Bumps version to 0.1.19.
Updates Changelog for 0.1.19.
Build 0.1.19
Merges branch 'release/0.1.19' into develop