Add alias support for select in SQL builder

[martok]

This adds table name aliasing capabilites to the Select::from() clause, using the same shorthand as in the various Select::join(). A simple test case is included.

This can be very useful for self-joins in tree-like structures, where otherwise columns would be ambiguous or require re-stating the table name very often. An example from my project, using the Record classes to avoid hard-coding table names:

$q->select(['a.id'])
  ->from(['a' => Inodes::table()])
  ->leftJoin(['b' => Inodes::table()], ['b.id' => 'a.parent_id'])
  ...

Result (wrapped for readability):

SELECT "a"."id"
FROM "inodes" AS "a"
LEFT JOIN "inodes" AS "b" ON ("b"."id" = "a"."parent_id")
...

[nicksagona]

Just merged it. Thanks for the contribution. FYI - Pop PHP v5 is close to being released, which includes the next version of this component, v6.