Implement io.Seeker for *sftp.File

Test that reads at the seeked offset of sftp.File are consistent with
the reads of offset of os.File for the same file.

Error conditions and undefined seeks are not tested.
Consistent puncutation, names and constants

  * European sentence punctuation, consistent with package docs
  * Match os.SEEK_* constants instead of numeric constants from io.Seeker
  * Consistent (shorter) receiver name for quickcheck value
Merge pull request #11 from streadway/seek

Implement io.Seeker for *sftp.File