ref #65: new formula functions IFNA and IFS and fix string compare result issue in arithmetic operations

- new formula function: YEARFRAC, ref #65
- update the codecov version
- remove unused variable

initialize formula function TRANSPOSE, ref #65 and remove unused exported error variable ErrToExcelTime

* Fix build-in scientific number format failed
* An error will be returned if given an invalid custom number format when creating a new style

- Close spreadsheet and row's iterator required
- New options `WorksheetUnzipMemLimit` have been added
- Improve streaming reading performance, memory usage decrease about 93.7%

- fix ineffectual variable assignments
- timeout in go test

- Fix formula percentages calculated incorrectly
- Make UpdateLinkedValue skip macro sheet
- Fix conditional format bottom N not working

- Support text comparison in the formula, also ref #65
- `GetCellValue`, `GetRows`, `GetCols`, `Rows` and `Cols` support to specify read cell with raw value, ref #621
- Add missing properties for the cell formula
- Update the unit test for the `CalcCellValue`

This closes #1012, support specify the formula in the data validation range, and update the documentation for the `AddPicture`

- Make unit test on Go 1.7
- API documentation updated

Co-authored-by: xuri <xuri.me@gmail.com>

Co-authored-by: NStani Michiels <git@rchtct.com>
Co-authored-by: xuri <xuri.me@gmail.com>