Added a javadoc comment that highlights that using HTTPS is restricted,
as coverartarchive.org only delivers HTTP links (not HTTPS) to images at
the moment.

Extended the API to query the URL from CoverArtImage, so the caller can
find out if the image URL are using HTTPS or not. This allows for taking
actions such as asking the user or trying to force HTTPS downloads, etc.
Increased compatibility with android.
- Replaced calls to EntityUtils.consume() with a shaded version
(HttpUtil.consumeEntity()) in order to be fully byte-code-compatible
with android's HttpClient.
- Introduced another constructor to DefaultCoverArtArchiveClient which
allows for setting a custom HttpClient, so the outdated version of
DefaultHttpClient() can be replaced.