Add and integrate client delegate protocol

These 'shouldHandleResponse' methods allow custom
handling of HTTP responses, ie, re-authenticating
upon 401's.
Expose the original request to client task completion handlers
Make the account model its client's delegate
Add delegation support to account model and add delegate protocol

An account requires a delegate. Also makes the
accounts manager an account delegate.
Present the accounts view if any request receives a 401

This happens instead of the default error handling
that displays a generic error alert.

Includes updating the accounts manager delegate
with a method that passes through this event.
Merge branch 'master' into peng/support-auto-reauth
Fixup sample app integration of 401s with showing accounts view

Ensure view is no longer represented as 'busy'.
Add client response-handling delegate methods tests

OCMProtocolMock allows the object to respond to
all protocol method selectors, even the optional
ones. This means there is additional boilerplate
to explicitly stub as many methods as needed to
reach the one we're testing.
Merge pull request #24 from 3dna/peng/support-auto-reauth

Support automatic reauthentication

Adds initial support for showing the accounts view
when any request gets a 401. This required
introducing much more additional delegation to
existing classes, mostly for the ability to stop
the default handling of requests in the event of
most types of errors encountered during the
process. The client's delegation support is backed
by tests that use newly introduced mocking and
stubbing from OCMock.