Move conditionally defined ndb helpers into their own module.

[dhermes]

Towards #433.

[dhermes]

@nathanielmanistaatgoogle PTAL, I addressed the comments. (UPDATE: Except for the one about string concat.)

[nathanielmanistaatgoogle]

You addressed it just fine; a counterargument is a perfectly valid form of addressing a comment. :-)

Looks great otherwise (if you really want to keep the as client_mod, well it's in an internal place so that's fine). Squash, retest, and merge. :-)

[dhermes]

Did the rename (UPDATE: specifically client_mod renamed back to client). Will squash and wait until the sun goes down for Travis