Use build_nested_hash to create hash trees

[elskwid]

Comments and code from: elskwid@a57b3b8

This implements build_nested_hash method that takes a block that returns a scope and makes a pretty nested hash. A few tweaks in this version,

• Returns an empty hash if a block is not given this is just a little clearer using return instead of the if
• Class method is a little noisier given we're working with the nested select
• Default limit_depth is removed as discussed here: elskwid@a57b3b8#commitcomment-2367578

[elskwid]

Updated and rebased against master. Just waiting on the ever-faithful-travis-workers.

[mceachen]

Right on. I'm trying to get the scope for hash_tree available to the test, because I want to assert that scope.size.should == scope.to_a.uniq.size (which is silly, sorta, but I think the query is scary enough to add this assertion.

If I make the scope a parameter passed to your new method, I can make my target class include a new module that lets me intercept the scope and make the assertion (or I could add a dev dependency on rr or mocha or something else and figure out how to make those do my bidding).

[mceachen]

OMG I LED YOU ASTRAY

Postgres takes a dump on the subselect, "column "depth" does not exist"

OK THAT'S IT LET'S SWITCH TO NOSQL.

[elskwid]

Text file support inbound.

[elskwid]

Annnnd, we're back.