Features/xmldecoder

This fixes #310 and adds a PayloadXmlDecoder to decode XML blobs.

I've also pulled out the Severity and TimestampLayout documentation for PayloadRegex, PayloadJsonand PayloadXml decoders so that it's consistent across all Payload*Decoders.  

ISO8601 timestamp layout is a new default for the PayloadRegexDecoder.

The moovweb/gokogiri XML parser is a binding over libxml2 which is tricky to compile on Windows and on OSX, you'll need Homebrew: https://github.com/mxcl/homebrew/issues/19030

Do we want to push this into a build-constraint so that we only run this on Linux?