icedtea7, icedtea7_web: New packages

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.

IcedTea-Web is a free implementation of the Java Web plugin and the Java Web Start programs.

The IcedTea 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 changesets (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
* A workaround was added due to Java security restrictions (explained in commit be8667627a060f9e4ee76751f9a703bcde2b3ebe )

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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 depends on PR #2017 (Python 3 support for lzma compression).

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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 to happen 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.