Write packager-friendly dependency policy

[clefru]

Grin adoption will also depend on how easy it is to install it and its dependencies. Dependencies have their own dependencies, which again have their own dependencies, which again.. You see where that goes. To not lose sanity there should be reasonable point in the dependency tree, where users can expect that their distributions just installs everything below.

To improve packagability and therefore installability of grin, I suggest to think about a dependency policy of the following form:
"Don't depend on X if it's not shipped in the most stable release Debian|Ubuntu LTS|Ubuntun|ArchLinux" (pick your favorite reference distro) or "Don't depend on X if it was released less than Y days ago".