adding cors support (and tiny bugfix)

cors support:
configure cors headers with require('sse-stream')({ path: '/sse', cors: {
  origin: 'someaddress.com'
  credentials: true
}})

see https://www.npmjs.org/package/access-control for full list options, default options
are as liberal as possible, e.g.  require('sse-stream')('/') will set cors options to 
   {origin: '*', credentials: false}  

There's no object extending, if a cors config object is supplied it overwrites the default
object, in keeping with the approach for the options object. 

the bug fix:
data coming through write method is a buffer, 
buffers (v0.10.30) don't have a split method, so process crashes, 
so we stringify the data as it's coming through.