Initial accounts view controller iPad support and sample app integration

Add convenience method for presenting accounts
view controller to reduce required app delegate
code. Also add integration with conventional
dismissal method.

Only show compact account button on iPhone.

Present as popover on iPad, without the close button.
Remove unnecessary accounts view controller delegate protocol
Minor sample app delegate reorganization
Simplify and reduce sample app delegate logic

Group together the required accounts aspect code.
Merge pull request #21 from 3dna/peng/accounts-widget-ipad

iPad support for accounts widget

Presents the accounts view as a popover instead of
a modal through adding convenience methods to the
library to help establish convention (their use is
suggested, but optional). Simplify the sample
app's app delegate logic, so it's clearer and
easier to copy from.