Add priority levels to observers

Observers can now take priority levels, which allow the user to control
the order of execution. Note that reactive expressions do not have
priority levels; since they are lazily evaluated, it wouldn't make any
sense to speak of priorities.

Another commit will be needed to add an API for changing the priorities
of outputs (probably in outputOptions?).
Allow integers or numerics for priority
Add priority to outputOption
Fix submit button interactions with tabs, plot sizes
selectInput: correctly handle choices with duplicate names. Fixes #157
Merge pull request #156 from jcheng5/fix-submit-button

Fix submit button interactions with tabs, plot sizes
Merge pull request #149 from jcheng5/observer-priorities

Add priority levels to observers
Update NEWS
Fixes for R CMD check
Bump httpuv version dependency
invalidateLater: don't invalidate when session closed
Merge pull request #159 from wch/session-invalidate

invalidateLater: don't invalidate when session closed
reactiveTimer: don't invalidate when session closed
Merge pull request #160 from wch/reactive-timer

reactiveTimer: don't invalidate when session closed
Run apps without creating files on disk

With this commit, runApp can now accept a list instead of a path.
This list should have elements named "ui" and "server" that contain
what would normally go in shinyUI and shinyServer, respectively.
(Note that there is no equivalent to global.R, nor should there
need to be since you can just directly execute in the global env
before calling runApp.)

Example:

runApp(list(
  ui = bootstrapPage(
    numericInput('n', 'Number of obs', 100),
    plotOutput('plot')
  ),
  server = function(input, output) {
    output$plot <- renderPlot({ hist(runif(input$n)) })
  }
))
Add basicPage function; like bootstrapPage with padding
Export basicPage
Fix rendering issues when slider gets too wide

Synced to jslider commit (from rstudio/jslider fork):
6de6ef7b2c788cbcec74dd51e0008e12247e6638
Initialize slider at correct time
Merge pull request #163 from jcheng5/app-object

Run apps without creating files on disk