new feature: protect sheet support, relate issue #273

Fix the issue caused by missing tradition to strict conversion for `sharedStringsReader()`, relate issue #276

* add new exported function to get Excel file content buffer

* refactor the WriteTo function

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

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

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

- godoc and go test has been updated

- 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

- 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

- New function `SetSheetRow()` has been added for support write whole line at once, relate issue #96 and #194;
- go test and godoc updated;
- Note that this function performance has not been tested

- godoc and go test has been updated

- Update the function `AddChart()`: clustered, stacked and 100% stacked bar and column series charts supported, relate issue #190;
- go test and godoc has been updated

- go test and document has been updated.

- go test updated

- go test updated

- godoc updated

- go test and 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