Merge pull request #25 from larshesel/vp

Add ReplaceSeries and refactor the CreateSeries function into smaller steps
CreateSeries now errors out if given duplicate keys

Also adds a new test to check most possible errors returned by CreateSeries
Merge pull request #28 from gsalgado/vp

CreateSeries now errors out if given duplicate keys
Add more error codes and messages, refine some doc comments.
Merge pull request #24 from teknico/vp

Add more error codes and message, reflow some doc comments.
Make ReplaceExistingSeriesTest table driven