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
```

1 files changed, 0 insertions, 12 deletions

diff --git a/lib.go b/lib.go
index fc95b23..4379be4 100644
--- a/lib.go
+++ b/lib.go
@@ -3,7 +3,6 @@ package excelize
 import (
 	"archive/zip"
 	"bytes"
-	"encoding/gob"
 	"io"
 	"log"
 	"math"
@@ -115,17 +114,6 @@ func intOnlyMapF(rune rune) rune {
 	return -1
 }
 
-// deepCopy provides method to creates a deep copy of whatever is passed to it
-// and returns the copy in an interface. The returned value will need to be
-// asserted to the correct type.
-func deepCopy(dst, src interface{}) error {
-	var buf bytes.Buffer
-	if err := gob.NewEncoder(&buf).Encode(src); err != nil {
-		return err
-	}
-	return gob.NewDecoder(bytes.NewBuffer(buf.Bytes())).Decode(dst)
-}
-
 // boolPtr returns a pointer to a bool with the given value.
 func boolPtr(b bool) *bool { return &b }