[FIX] Top-aligned popovers: Arrow border-color
[FIX] Popover directive: Make rpPopoverTitle optional
[FEATURE] Popover directive: Add rpPopoverContent

The value of this HTML attribute is parsed for Angular expressions
in mustaches ($interpolate). Use rp-popover-content if dynamic data
is needed in the popover, keep data-content otherwise (no parsing then).

@vhpoet
[FEATURE] Popover directive: Add rpPopoverDelay

If specified, wait for this delay in milliseconds before showing the
popover.
[FEATURE] Balances widget: Add balance popovers

The popover contents shows the balance in exact precision. The popover
content is wrapped in a localizable attribute (l10n-rp-popover-content)
as certain languages may have the currency symbol in front of amount.
[FIX] Balances widget: Balance popovers positioning

Also set max-width for them.