Alert directive styling, expansion, usage

The changes (3 changes, 3 commits):
1. Styled the rest of the alerts: info, warning, danger. (https://github.com/stellar/stellar-client/pull/733 only implemented it for success)
2. Added option to the `alert` directive to put a click handler on the dismiss button
3. Converted old code to use the new alert directive.
if dismissable is true, it gets a default implementation... see below