workBookRelsWriter, drawingRelsWriter into relsWriter;
drawingRelsReader, workbookRelsReader, workSheetRelsReader into relsReader;
addDrawingRelationships, addSheetRelationships into addRels

- Testing case for set and get doc properties
- Update charts struct XML tags

- go test and godoc has been updated

- go test and document has been updated.

- Support insert new lines into shape, relate issue #38, note that the format set parameter of function `AddShape()` changed;
- go test and go doc updated

- Add a donation link, related issue #57  ^_^

- Drawing `nvPicPr` element ID property calculation changed;
- go test updated

- go test updated

- Doc for function `GetRows()` updated, relate issue #43;
- Fix doc typo in `xmlContentTypes.go`;
- Default template updated;
- Readme updated;
- go test updated

- go test updated

- Partial logic performance optimization, use pointer reference instead of a pass the variable value;
- Add comments for content types struct definition;
- Update go test `TestSetBorder` section

- Function parameter code is simplified;
- Fix element `Tint` value parsing error in worksheet;
- Update go test