Fix documentation

File::Slurp isn't required since v0.015 but it was still being
mentioned in the installation instructions.
Remove 'push' options from the documentation headings

I substituted the generic "OPTIONS*" for the explicit list of options
from the section heading of the push sub-command because Pod::HTML
wasn't producing the index entry for 'git gerrit push', since it was
too large.

Now the complete list of options for push can be seen at a glance in
the SYNOPSIS and also in their documention in each sub-command
section.
Remove dead code

Remove two routines that weren't used anymore.
Reorder some routine definitions

Just to define them in the same order they're documented.
Rename App::GitGerrit to Git::Gerrit

I decided to rename the module back to its original name because I
couldn't have it indexed on CPAN. Since version v0.010 I started
having problems with the CPAN indexing. The last attempt at solving
can be read at
http://www.nntp.perl.org/group/perl.modules/2013/10/msg87890.html.

I'm renaming it hoping that this way I can have it properly indexed at
CPAN.
v0.018

  [Changes]

  - Rename App::GitGerrit back to its original name: Git::Gerrit. This
    is an attempt to bypass a problem that is preventing the module
    from being indexed at CPAN.

  - A few changes in the documentation.