Optional iso dates (#94)

* Optionally use ISO formatting for dates

Prior to this commit we always used the date pattern as
assigned in the Excel workbook for the cell. Which could
lead to surprising results.

We now allow the dates to be formatted as an ISO date time with
offset, so dates/times are consistently formatted in the result.

Closes: #88

spring-batch-excel/README.adoc

@@ -91,6 +91,7 @@ public RowMapper rowMapper() {
 | `rowSetFactory` | no | `DefaultRowSetFactory` | For reading rows a `RowSet` abstraction is used. To construct a `RowSet` for the current `Sheet` a `RowSetFactory` is needed. The `DefaultRowSetFactory` constructs a `DefaultRowSet` and `DefaultRowSetMetaData`. For construction of the latter a `ColumnNameExtractor` is needed. At the moment there are 2 implementations
 | `skippedRowsCallback` | no | `null` | When rows are skipped an optional `RowCallbackHandler` is called with the skipped row. This comes in handy when one needs to write the skipped rows to another file or create some logging.
 | `strict` | no | `true` | This controls wether or not an exception is thrown if the file doesn't exists or isn't readable, by default an exception will be thrown.
+| `datesAsIso` | no | `false` | Controls if dates need to be parsed as ISO or to use the format as specified in the excel sheet. *NOTE:* Only for the `PoiItemReader` **not** the `StreamingXlsxReader`!
 |===
 
  - `StaticColumnNameExtractor` uses a preset list of column names.