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

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

- go test and godoc has been updated

- Update the function `NewSheet()` to handle when already exists a worksheet of the same name, relate pull request #196;
- go test and godoc has been updated

- godoc 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

- go test updated

Adds a '{' to the example for sheet.GetSheetMap()
Signed-off-by: NMark Stenglein <mark@stengle.in>

- 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

- Drawing `nvPicPr` element ID property calculation changed;
- go test updated

- go test updated

- Typo fixed;
- go test updated

Fix return index error in function `GetSheetMap()`;
Fix return worksheet name error in function `GetSheetName()`;
go test updated

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

- go test updated

- go test updated