HttpFeed authentication and AS7 security

* Adds support for HTTP authentication schemes to `HttpFeed`. It should support all the schemes supported by httpcomponents (basic, digest, NTLM, SPNEGO and Kerberos), but things like NTLM probably need more configuration. See `HttpFeed.Builder.credentials(username, password)`.
* Alters feeds/pollers to add `checkSuccess`, `onFailure` and `onException` methods.
* Enables security for the AS7 management interface by default. Set `MANAGEMENT_USER` and `MANAGEMENT_PASSWORD` config keys. Defaults are 'brooklyn' and a random UUID.
* Adds `IntegerConfigKey` class and (String name, String description) constructors to String/BooleanConfigKey.
* Adds optional basic authentication to `HttpService` test utility class.

Fixes issue #785.