Add package to Linux package managers

[jasonmp85]

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).

[jasonmp85]

So pg_shard is in the Yum repositories now, but I don't know what work is needed to get it into the Apt ones… I'll ask around.