Update grid.base.js

Improving performance by
 1) removing setting of `ts.grid.bDiv.scrollTop = 0;` which take many time in case or many rows inserted, but it will be called **always** after adding rows in *empty* grid, so `scrollTop` is already 0.
 2) `.append(rowData.join(''))` make many common validations and take many time in case of many rows inserted. The method `.append` works **not only with string values** so such implementation is OK. We have **pure string value** which need be appended to the `<tbody>`, so we can improve the performance by usage `+=` on `innerHTML` of the `<tbody>`. REMARK: the suggested line will be applied on almost **empty grid**. The grid contains only the first line with `tr.jqgfirstrow`. So no memory leaks are expected.

More information about the changes are described [here](http://www.trirand.com/blog/?page_id=393/bugs/improving-performance-of-addjsondata)