Function `GetCellValue()` performance improvement by avoid repeating deserialization, relate issue #70.

- Unify the index row number index of functions `SetRowHeight()` and `GetRowHeight()` relate issue #68;
- Unify the return value data type of functions  `SetColWidth()` and `GetColWidth()`;
- 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

- go test updated

- Function `SetCellValue()` support `time.Time` data type parameter, relate issue #49;
- go doc and go test updated

- Function `GetRows()` doc updated, relate issue #43;
- go test and embed template updated

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

- 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

- Add escape characters of sheet name;
- Update go test and fix typo