Added Visible State support

VisibleState is now a property on IExcelDataReader as well as the Xls
and Xlsx document classes.
VisibleState can be accessed from a DataTable produced by AsDataSet() by
using extended property "visiblestate".
ExcelOpenXmlReader remove check applyNumberFormat when deciding if to attempt data conversion. closes #19
Merge pull request #34 from SteveLillis/master

Added Visible State support
Merge branch 'master' of github.com:ExcelDataReader/ExcelDataReader

Conflicts:
	Excel.Tests/ExcelBinaryReaderTest.cs
Fix oen office test broken by last change
Share core format between portable and .net 2
Merge remote-tracking branch 'origin/master' into f-portable

Conflicts:
	Excel.Test.4.5.sln
Changes from pull request =into portable library.

Test namespaces altered so both .net2 and 4.5 can run.