Add possibility for using shared account for gemfury via command line
Minor style and test fixes for `--as` argument.

  * Rename the test so it doesn't mention Gemfury (which the test itself
    doesn't use)
  * Add another test for the long-form option, for completeness' sake
  * Remove the extra empty string passed to `add_option`; because of the
    way the `add_option` method works (namely by taking an arbitrary
    list of arguments and attempting to divine their meaning), this extra
    argument isn't necessary
Merge pull request #23 from alphagov/shared-account-plugin-argument

Allow Gemfury shared accounts in the Rubygems plugin