Get value instead of formula on ExcelSheet::read()

I had the problem, that when reading an Excel sheet, your binding returned the formula instead of the value itself. I copy/pasted the stuff together, so normally, always the value is given instead of the formula (which is the more common use case in my opinion). Imho, it should (really?) solve [Issue 48](https://github.com/iliaal/php_excel/issues/48).

I also added a new method `ExcelSheet::readFormula()` which will behave just as `ExcelSheet::read()` did before. It will give the formula if the cell has one assigned, or the value otherwise. `ExcelSheet::readCol()` and `ExcelSheet::readRow()` will use the new behavior of `ExcelSheet::read()`. **Attention! This may cause incompatibilities to older versions!**

I am pretty sure this can be written in a better fashion, but it is working for me...

BR, Georg.