Provide feedback to the user (fixes #7).

* Actually display an error message when a request is not
  successful.
* Show a message when we are waiting for the server.
* Make clear the program ended.
fix sample code
inherit the font family for status messages
allow setting the version via a query parameter
include the version in the shared url
follow the native JavaScript naming style
update for 0.11.0
whitelist pread64
Revert "simplify the sandboxed helper scripts"

This reverts commit c6765e7128ed872ea7c69fd560b8cf7042755b33.
Add nanosleep to whitelist

The following program doesn't work in play.rust-lang.org:

    fn main() {std::io::Timer::new().unwrap().sleep(1000);}

Running on my machine under strace, it appears the only disallowed syscall is nanosleep.  I don't see a reason it should be prohibited, so I've added it to the whitelist.
Allow the evaluate end-point to separate output from compiler vs. program.

Specifying separate_output=true (in JSON) in an evaluate request will
mean the return value is then `{ "rustc": <output of the compiler>,
"program": <output of the program> }`.
move decoding out of the execute API