Update to Arel::Table.alias method

[ozzyaaron]

Hi,

A colleague and I were using the Table.alias method today to do multiple joins on the same table but it wasn't working how we expected out of the box according to the readme page.

I've just updated the method to auto-increment the alias name if none is provided, otherwise the user can specify the name of the alias as they did previously.

I hope this is worthwhile :)

Thanks Aaron