Update README

I'm taking out the warning message about this being beta code. I'm
using it for a while and a few of my colleagues are too.
Make options defaults configurable

Now you can specify a default set of options to be passed to any
sub-command by setting a variable called 'options.<command>'.
Refactor

Interpolates the $Command variable in error messages instead of using
the sub-command name explicitly.
Remove dependency on File::Slurp

It's a pity that File::Slurp still doesn't support encoding
specifications. See
https://rt.cpan.org/Public/Bug/Display.html?id=34444.

Until it does I can't use it.
v0.015

  [New features]
  - It's possible now to specify a default set of options for each
    sub-command using the variable git-gerrit.options.COMMAND.

  [Changes]
  - Take off the message about this being beta code from README.

  [Fixes]
  - Remove dependency from File::Slurp in order to correctly support
    locale-based encodings in the message file created by the editor.