Add DataStore interface and basic implementations
Share a common DataStore through XProcRuntime
Use DataStore for p:data and non-xml p:document
Use DataStore in p:store, non-http p:http-request, text p:xinclude, and result of p:xsl-formatter
Use DataStore for basic fileutil operations
Use DataStore for zip/unzip, java-properties, and css-formatter steps
Merge pull request #2 from jamesrdf/datastore

Datastore