Changed splash screen, --version messages and comments (VIM to NeoVIM).

An attempt at #92 which

* replaces "VIM - Vi IMproved" with "NeoVIM - New Vi IMproved"
* resets all version specifiers to 0 and removes all patch levels
* adds "the http://neovim.org/ team." as MODIFIED_BY to be displayed on splash screen.

Wording is up in the air, but the vim license explicitly requires we differentiate ourselves from the main distribution, i.e. by defining MODIFIED_BY.