v0.5.0 - More Password Customization & Bug Fixes

* Allow setting the `password-alignment` option to `center`, in addition to
  `left` or `right`.
* Add a `password-border-radius` configuration option to set the border radius
  of the password input.
* Add a `password-character` configuration option to set the character used to
  mask the entered password. `-1` specifies the default character, `0` hides
  any entered text, & the first character is used for any other value.
* Add a `password-input-width` configuration option to set the number of
  characters that should fit in the password input. This is just a suggested
  width - GTK may render a narrower input.
* Add a `show-image-on-all-monitors` configuration option to display the
  background image on every monitor instead of just the primary monitor.

* Ensure the mouse cursor is always hidden.
* Disable the password input while authenticating. This fixes a bug causing
  lightdm to crash if the greeter tried authenticating before a previous
  authentication attempt had completed.
* Ignore any trailing whitespace in the `user`, `mod-key`, &
  `password-alignment` configuration options.

* Improve error message when configuration file is unreadable.
* Document how to make configuration file a symlink into home directories.