Fixes #136: I18N support for Windows

Currently I'm forcing UTF-8. Normally this is not an issue for Linux/Mac users. For Windows users, their system default encoding is not UTF-8, but it should be fairly easy for them to re-save their ui.R/server.R/etc with the UTF-8 encoding (e.g. the File menu in RStudio IDE); by comparison, I feel it is hairy to carry an `encoding` argument everywhere that may take different values, e.g. in `runApp()`. Another reason to force UTF-8 is ShinyApps.io -- if a Windows user wrote an app in, say, ISO-8859-2 or GB2312, it will not work when published to ShinyApps because the locale will be different there, which may be surprising if he/she is not familiar with the encoding issue.

Pandoc was brave enough to accept UTF-8 input only, and it seems there are not people shouting at it... Let's see if shiny Windows users will yell at us :)

Eventually we will write an article on shiny.rstudio.com explaining this topic. For example, when using read.csv(), the `fileEncoding` argument may be explicitly specified for the sake of portability.