data validation drop-down list use sqref cell  issue #268

 Initialize data validation support

Change `allowBlank`, `ShowErrorMessage` and `ShowInputMessage` type as boolean, add new field `ShowDropDown`, change fields orders follow as ECMA-376-1:2016 18.3.1.32.

Issue templates have been added.

Use min/mid/max value for 2 and 3 color scale conditional formatting

added retrieval of worksheet comments

- RGBA, HSL color convert has been added;
- go test updated

- Fix doc typo

Extract WriteTo method (see io.WriterTo) to expose bytes written

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
```

Extract a WriteTo() method (see io.WriterTo) that exposes the count of bytes
written and rewrite Write() to use it.

- 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