Add package to Linux package managers

Though #67 will take care of serious PostgreSQL users, many will prefer to install `pg_shard` using OS package management tools (`apt`, etc.). Many PostgreSQL add-ons are included in Ubuntu's built-in repositories (Slony-I shows up as as `postgresql-9.3-slony1-2`, for instance). If we could get `pg_shard` in some of these repositories it could make building cloud instances easier (with CloudInit scripts or similar).

At any rate, users might appreciate an install that doesn't require a compilation step (even if it is hidden within PGXN client or `pex`).