Fix compile error under Qt 5.0

QStringRef::toInt() and QStringRef::toDouble() are introduced in Qt 5.1
Change static member of SharedStrings to normal member
Fix Issue 7: Cell string misplacement when rich text exist

A private class RichString has been added. More work is needed
Issue 9: Update README file
Don't lost rich text information
Code refactor: Add prefix "XlsxFormat" to some internal class
Code refactor: Make use of QSharedDataPointer for Format
Code refactor: numFmt part of QXlsx::Format
Code refactor: Remove private class XlsxFormatFontData
Code refactor: Remove private class XlsxFormatBorderData
Code refactor: Remove private class XlsxFormatFillData
Code refactor: Remove private class XlsxFormatAlignmentData and
XlsxFormatProtectionData
RichString: Using Format instead of Format *
API change: Using Format instead of Format *
Code refactor: Only create FormatPrivate when needed.
Update to Version 0.2.0
Make RichString public
Format: Use QMap instead of QHash

As the items of QHash are arbitrarily ordered, so the same formats may generate
different formatKey
Update documentation
Add internal support for dxf format

test needed.