improved comment formatting

make the comment box's height auto and having the minimum height.

Restore date 32bit compatibility

Do not use large int64 constants that are not available in GOARCH=386
Fix #239

Ability to parse dates further in future

Golangs time.Duration uses nanoseconds, thus it is limited to approximately 290 years.

CopySheet() using reflect instead of encoding/gob

Use github.com/mohae/deepcopy to deep copy worksheets instead of the
internal deepcopy function that was using encoding/gob serialization and
deserialization.

Rationale:
1/ using `encoding/gob` is much slower than
   [`mohae/deepcopy`](https://github.com/mohae/deepcopy/)
2/ When building an application this implementation of `deepcopy` drags
   the `encoding/gob` package into the binary. And this package is much
   bigger than `mohae/deepcopy` (which only depends on `time` and `reflect`).

```
$ LC_ALL=C stat -f "%6z %N" $(go env GOPATH)/pkg/$(go env GOOS)_$(go env GOARCH)/github.com/mohae/deepcopy.a $(go env GOROOT)/pkg/$(go env GOOS)_$(go env GOARCH)/encoding/gob.a
 10508 .../pkg/darwin_amd64/github.com/mohae/deepcopy.a
541818 .../pkg/darwin_amd64/encoding/gob.a
```

- Add error return value for functions: `AddChart()`, `AddComment()`, `AddPicture()`, `AddShape()`, `AddTable()` and `SetConditionalFormat()`
- go test has been updated

- go test and godoc has been updated

- Using the canonical syntax in issue template and contributing guide;
- go test has been updated

Added helper functions to set the chart size.

Signed-off-by: NEugene Dzhurinsky <jdevelop@gmail.com>

Added functions to set and get outline level for columns and rows.

Added helper functions to set the chart size.
Added the unit test
Signed-off-by: NEugene Dzhurinsky <jdevelop@gmail.com>

- Remove useless function `replaceWorkSheetsRelationshipsNameSpace()`;
- Make test cases use strict error checking

save bytes on memory instead of string

Added ZoomScale SheetViewOption

Add iterator method for rows

- godoc and go test has been updated

- go test and godoc has been updated

Accessible value between 10 - 400

Used as:
xlsx.SetSheetViewOptions(sheet, 0, excelize.ZoomScale(75))

Accessible value between 10 - 400

Used as:
xlsx.SetSheetViewOptions(sheet, 0, excelize.ZoomScale(75))

- go test and godoc has been updated

- Fix `checkCellInArea()` index out of range when merged cell ref is single coordinate, relate issue #206;
- go test updated;
- Repository icon for SourceTree has been added

- go test and godoc has been updated

- Add categories or values on reverse order (orientation of the chart) support, and set auto or fixed maximum, minimum of the axis, relate issue #202;
- go test and godoc has been updated