Rename filestore

Follow on from #8.
make s3 the default choice.
rename wrappers to FilestoreManagers (and bring naming in line with the rest)
remove S3 references from function names and comments outside of S3FilestoreManager (+test)

Not sure why github spotted the s3Wrapper renames and not the fsWrapper renames. Presumably enough changed lines from the dereferences to make it think it was a whole new file.