Added foo:install:assets task that copies assets from plugins public directory to application's public directory

This is the most simple and naive approach: just copy every files from engine to app.
The only exception is when file has changed, in that case developer will be
asked if he wants to rename the field.

There is no need to make this task more sophisticated as 3.1 will be shipped
with better assets handling and it will be the default way to handle things.

This reverts commit 9c53e82d.

	This makes a test fail, but it is revealing a bug in Arel master.

Refactor JoinDependency and friends so that a JoinAssociation can produce an arbitrary number of joins, which will be needed in order to support nested through associations.

Add some tests for functionality in JoinAssociation which already exists but was previously untested