add a Note section to the documentation of conditionalPanel(), since it always confuses R users

e.g. https://groups.google.com/d/msg/shiny-discuss/AFItYcRXzyw/ywRy3EEtjw4J
Merge pull request #539 from yihui/doc/conditionalPanel

add a Note section to the documentation of conditionalPanel(), since it ...
convert the message to native encoding from UTF-8 before decoding it

on Windows, switch(input$foo, foo1 = val1) does not work even if input$foo ==
foo1 but Encoding(input$foo) is UTF-8 while foo1 is unknown (native encoding);
to prevent such problems, let's always use native encoding inside the shiny
process, and only do the UTF-8 conversion at I/O time

special thanks to @desar for the thorough tests
let's read ui.R, server.R, README.md, and DESCRIPTION also with UTF-8

the reason for this is that htmltools::htmlEscape() uses gsub(..., x, fixed =
TRUE), which does not work on Windows if x is encoded in UTF-8; fixed = TRUE
only works with the native encoding
json2.js modified and uglified from https://github.com/yihui/JSON-js via http://lisperator.net/uglifyjs/

I'm using the date of json2 as its version number, since it does not really have a version number

This should close rstudio/shiny-server#79