add AnnotationSeries datatype

[stephprince]

Add the AnnotationSeries datatype to hold text information from the acquisition system.

Follow the general steps for adding a new data type:

• add a class for AnnotationSeries
• add a createAnnotationSeries function on NWBFile
• add a writeAnnotationData function on recordingContainers
• add a writeText or relevant convenience function on AnnotationSeries if needed