MailEntityProcessor Update

This patch addresses a number of issues in the MailEntityProcessor contrib-extras module.

The changes are outlined here:

• Added an 'includeContent' entity attribute to allow specifying content to be included independently of processing attachments
  e.g. <entity includeContent="true" processAttachments="false" . . . /> would include message content, but not attachment content
• Added a synonym called 'processAttachments', which is synonymous to the mis-spelled (and singular) 'processAttachement' property. This property functions the same as processAttachement. Default= 'true' - if either is false, then attachments are not processed. Note that only one of these should really be specified in a given <entity> tag.
• Added a FLAGS.NONE value, so that if an email has no flags (i.e. it is unread, not deleted etc.), there is still a property value stored in the 'flags' field (the value is the string "none")
  Note: there is a potential backward compat issue with FLAGS.NONE for clients that expect the absence of the 'flags' field to mean 'Not read'. I'm calculating this would be extremely rare, and is inadviasable in any case as user flags can be arbitrarily set, so fixing it up now will ensure future client access will be consistent.
• The folder name of an email is now included as a field called 'folder' (e.g. folder=INBOX.Sent). This is quite handy in search/post-indexing processing
• The addPartToDocument() method that processes attachments is significantly re-written, as there looked to be no real way the existing code would ever actually process attachment content and add it to the row data

Tested on the 3.x trunk with a number of popular imap servers.