GEOMESA 325: allow ingest without date information

* date information flags like dtformat and dtfield are now optional.
* added tests to check that writing the features worked.
* updated readme.
I think you need to invert this map a bit:

```scala
def buildDtBuilder: Option[(AnyRef) => DateTime] = 
   dtgField.map { f =>
      val attrCandidate = attributes.find(_.getLocalName == f)
      attrCandidate match {
         case attr ....
      }
   }
```