Fix subtle name resolution bugs

See in particular:
http://stackoverflow.com/questions/13575353/how-does-the-shiny-r-package-deal-with-data-frames

Also reported at different times by Dirk Eddelbuettel and Jay Emerson.

The observed behavior is that S3/S4 method dispatch does not always seem to
work; the desired methods are not invoked despite appearing to be in the
search path.

The problem was that sourcing files with local=TRUE creates a new environment
based on the parent frame, which in our case is Shiny's package environment.
What we really want is to read from the global environment but write to a
throwaway environment. The correct way to do that is to make a new environment
with .GlobalEnv as the parent.

R/shiny.R

@@ -412,7 +412,7 @@ dynamicHandler <- function(filePath, dependencyFiles=filePath) {
       if (file.exists(filePath)) {
         local({
           cacheContext$with(function() {
-            source(filePath, local=TRUE)
+            source(filePath, local=new.env(parent=.GlobalEnv))
           })
         })
       }
@@ -659,7 +659,7 @@ startApp <- function(port=8101L) {
   serverFileTimestamp <- NULL
   local({
     serverFileTimestamp <<- file.info(serverR)$mtime
-    source(serverR, local=TRUE)
+    source(serverR, local=new.env(parent=.GlobalEnv))
     if (is.null(.globals$server))
       stop("No server was defined in server.R")
   })
@@ -733,7 +733,7 @@ startApp <- function(port=8101L) {
           shinyServer(NULL)
           local({
             serverFileTimestamp <<- mtime
-            source(serverR, local=TRUE)
+            source(serverR, local=new.env(parent=.GlobalEnv))
             if (is.null(.globals$server))
               stop("No server was defined in server.R")
           })