icedtea7: New package

IcedTea is a project to build OpenJDK using Free Software tools. It is updated frequently with security fixes and it has support for additional features, compared to vanilla OpenJDK.

The derivation is very similar to the OpenJDK derivation, except for the following notable changes:
* The code seems to be separated into a bunch of software bundles, which need to be downloaded separately and need to be taken from specific Mercurial commits (which are specified inside the IcedTea tarball).
* The patching of the OpenJDK code (by Nix) must be done after extracting the OpenJDK bundle, which happens during the build process

Updating this derivation can be rather cumbersome, as you need to update the ```sources.nix``` file to point to the correct changesets, which are specified in the ```Makefile.am``` and ```hotspot.map``` files inside the IcedTea tarball.

To facilitate the update process, I have included a Python script which does the entire job of checking for an update, downloading the IcedTea tarball and update the ```sources.nix``` file with the correct versions, changesets and checksums.

The update script needs depends on PR #2017 (Python 3 support for lzma compression).

Taking into account that OpenJDK has a bunch of security vulnerabilities, I would really like for us to remove it, or at least not use it by default, and use IcedTea instead.
But for that I think we will need to first create a bootstrap version of IcedTea, so that it can build itself without depending on vanilla OpenJDK... so I think that probably belongs in a separate PR.