escape HTML entities to avoid XSS attacks in DataTables

BTW, per recommendation of http://datatables.net/manual/server-side, the parameter `draw` is coerced to integer

A test case:

```r
library(shiny)

shinyApp(
  ui = fluidPage(
    h1('Escape everything'),
    dataTableOutput("table1"),
    h1('Escape nothing'),
    dataTableOutput("table2"),
    h1('Do not escape the first column'),
    dataTableOutput("table3")
  ),
  server = function(input, output) {
    testdata <- rbind(
      iris[1:29, 1:2],
      c('<b>Bold Font Here</b>', '<script>alert("hello!")</script>')
    )
    colnames(testdata)[1] <- as.character(em(colnames(testdata)[1]))
    output$table1 <- renderDataTable(testdata,
                                     options = list(pageLength = 10))
    output$table2 <- renderDataTable(testdata, escape = FALSE,
                                     options = list(pageLength = 10))
    output$table3 <- renderDataTable(testdata, escape = -1,
                                     options = list(pageLength = 10))
  }
)
```