HTTP proxy support

By default RssRiver does not use a proxy host or a proxy port.
But it would be helpful to be able to setup a dedicated proxyhost/port
for this plugin.
RSS/Atom custom fields

Rome provides RSS custom fields as 'ForeignMarkups'.
RssToJson.java uses this facilty to create inner objects (named using
namespace prefix) containning  every custom fields.
updateDate and content fields support

Added updatedate and content mapping.
RRS/Atom custom fields : mapping to JSON value types

Rome foreignMarkups are 'Strings'.
RssToJson tries to parse these strings into JSON 'types'.
Use rss:guid or atom:id as elasticsearch id field

If rss guid or atom id is not null (provided by Rome SyndEntry.getUri);
we use it as an id field.
Use explicit  "_lastupdate_" + feedname status messages (instead of UUID generated from url)

This changes make it's quite easier for 'human' readers to check last
udpade dates related to feeds.
In order to be able to use feedname, these field is now mandatory and
unique within an index.
Revert "Use explicit  "_lastupdate_" + feedname status messages (instead of UUID generated from url)"

This reverts commit a63c5dbaae4a2ea7bacdc1f5c3ec6da1d9682579.
Explicit "_lastupdate_"  Ids (by using feedname)

This changes make it's quite easier fro 'human' reader to check last
update related to feeds.
In case feedname is not provided, url is used.