Reduce allocations when writing

Fix #494

If a row is full, don't bother allocating a new one, just return it.

Use the last populated row as a hint for the size of new rows.

Simplify checkSheet to remove row map

* Added accessors for Getting/Setting Page Margins

* Added test cases

solve ending space missing

Add missing ShowZeros SheetViewOption implementation

* Allow access to more formula attributes in SetCellFormula

Make SetCellFormula variadic to not break API.
The new arguments are option arguments in which the type of
the formula and the ref attribute may be set.
These need to be set for an array formula to work.

* Add TestWriteArrayFormula to test optional parameters of SetCellFormula

TestWriteArrayFormula writes a document to the test directory that
contains array formulas that are used to calculate standard deviations.
The file also contains values calculated by the Go testcase, so the
results can be verified. It should be tested, if the array formula
works (i.e. shows a number, not an error) and that the values calculated
by the formula and those calculated by Go are the same.

workBookRelsWriter, drawingRelsWriter into relsWriter;
drawingRelsReader, workbookRelsReader, workSheetRelsReader into relsReader;
addDrawingRelationships, addSheetRelationships into addRels

Fix dependency on github.com/360EntSecGroup-Skylar/excelize v1

* #386 regression test added

* closes #386 string to bigint on GOARCH=386

Updating Readme

Removing the /v2 on the package url as it does not work with the ```go get``` command.

add missing error check in SetSheetRow()

SetCellValue: use fmt.Sprint(v) instead of fmt.Sprintf("%v", v)

Because that does the same thing, but without having to parse a format
string.
Signed-off-by: NOlivier Mengué <dolmen@cpan.org>