- go test and document has been updated.

- godoc updated

Signed-off-by: Ri Xu <xuri.me@gmail.com>

- API changed, use worksheet name instead of "sheet" + index, related issue #25, #43, #47, #51, #89, #101, #116 and #120.
- go test updated

- Readme and go test updated

- Make function `TitleToNumber()` exportable, note that function `ToAlphaString()` return value calculation changes, get more info from go doc. Relate issue #63;
- Readme and go doc updated

- Simplified code

- 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

- Optimize deserialization operations;
- README updated, add go version required notice

- go test updated

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

- Improved performance when reading large files, call Token to read tokens one by one instead Unmarshal. Related issue #20 ;
- Fix go test typo;
- Update README

- Character limits for cells added;
- Update go test and fix typo

- go test updated;
- fix typo and `gofmt -s` formatted