Add support for multiple replaygain backends

Issue #260 raises a problem with the current replaygain plugin where many formats are not supported.
GStreamer is a cross-platform library which is able to compute replaygain values for practically any known format. Unfortunately, it has quite a lot of dependencies and is very annoying to install on windows.

In order to keep both the current implementation, and add support for future, more robust implementations, a sort-of backend framework was needed for replaygain calculation.

This pull request implements both a simple replaygain calculation framework and a GStreamer implementation which works both for track mode and album mode.