add new subtle highlight effect for table on hover.

Added a new subtle highlight effect for tables (using the `pure-table-highlight` classname) when the mouse is `hover`-ed over the rows. I have also added the appropriate test in the `src/tables/tests/manual/tables.html` file. I have also added the appropriate documentation in the **pure-site** project. I'll be sending a pull request on the **pure-site** project shortly.

I have used the following CSS style rule with a hover color of `#f2f2f2`:
```
.pure-table-highlight tr:hover td {
    background-color: #f2f2f2;
}
```

Please let me know if this is correct.

Thank you!