Added the IDE category and Eclipse Luna framework.

Hi,

this is the first pass, and isn't ready to merge yet.

To download Eclipse, we use a direct link to file which redirects to the best mirror. This link is accessible from the right sidebar at https://www.eclipse.org/downloads/download.php?file=/technology/epp/downloads/release/luna/R/eclipse-standard-luna-R-linux-gtk-x86_64.tar.gz and I think is unlikely to change, so it's safe to hard-code. The MD5 is fetched similarly. Since we don't need to parse anything, I just use the first hook (download_provider_page) to do everything.

This works properly as far as I can tell. What is missing:

* the CLI status indicator doesn't change from 0%. Although for me this doesn't work for Eclipse-ADT either. Status updates are sent though (this is handled in DownloadCenter).
* there are no tests (yet)
* there is no support for 32-bit (yet)

I'll be away for a few days before I can continue working on this. Comments welcome!
I've moved recently the call to mark_in_config at the very end (maybe that should be part of the BaseInstaller directly, what do you think?

https://github.com/didrocks/ubuntu-developer-tools-center/commit/cd64c8868325f3dfb2814378f291354cc40b351b#diff-46a00461b5fcdd2675af2c4bb7b5b6d3R326

See the commit comments about why this change.