an edge case selectInput(choices = NULL)

firstChoice() may fail with an error "subscript out of bounds"
add links to examples in shiny dev center
a news item for the optgroup feature
warn against the byte order mark if exists; fixes #545
deal gracefully with elements that have no computed style available
update comment with more specific browser notes
Merge pull request #550 from rstudio/bugfix/offscreen-shinyapps

Deal gracefully with elements that have no computed style available
add explanations of skipping *nix when checking encoding, and point to the shiny dev center article
one more check before we use UTF-8: see if there are embedded nul's
Merge pull request #549 from yihui/bugfix/utf8-bom

warn against the byte order mark if exists; fixes #545