imap: support deleting messages
imap: support multipart w/o plaintext part
[syslog5424] confirm process ID is nil when RFC NILVALUE used
[syslog5424] structured data should be nil when RFC NILVALUE is used
[syslog5424] non-capturing parens since no backreferences needed
mutate: fix strip feature
Merge pull request #590 from jdve/fix-mutate-strip

mutate: fix strip feature
Merge pull request #591 from jdve/imap-support-delete

imap: support deleting messages
Merge pull request #588 from jdve/fix-imap-multipart

imap: support multipart w/o plaintext part
Merge pull request #586 from bfritz/syslog5424-fixes

nil instead of "-" for missing structured data with grok pattern for rfc 5424 syslog
[syslog5424] do not capture chevrons with priority value

Priority is "191", not "<191>".  The syslog_pri filter expects a
raw integer without the extra punctuation.
Merge pull request #593 from bfritz/syslog5424-fixes

[syslog5424] do not capture chevrons with priority value
- Make 'match' no longer required since logstash has generalized
  conditionals/branches to do that in a more general way.
- Tested with netcat and me pretending I'm a mail server. It was
  awesome.
- file uses plain codec, no \n hack necessary
use write instead of puts, again
- upgrade ES to 0.90.3
- exclude @timestamp and @version if 'fields_are_metrics' is set.
- add codec support
- message_format is deprecated
Add a trailling \n to the json encoding to keep backwards
compatibility to old behavior.
- use Numeric instead of Fixnum (covers Float, too)