Rename Git::Gerrit to App::GitGerrit

I changed my mind about the distribution name. I decided to use
App::GitGerrit after reading about App::Ack.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Mention Gerrit::REST in the documentation

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
v0.003

  - Rename git-gerrit's distribution from Git::Gerrit to
    App::GitGerrit.