Add test that runs every nannou example listed in the Cargo.toml
Add test that runs every nannou example listed in the Cargo.toml

This adds an example that runs each nannou example listed in the
Cargo.toml for a few seconds each, one at a time.

This is useful for finding `panic!`s that only occur at runtime, as
while `cargo test` does compile the examples, it does not try and
execute them.

The test has the `#[ignore]` attribute so that `cargo test` does not run
the example by default. This is because 1. the test takes a looong time
to run and 2. travis is likely incapable of running a lot of the tests
as it doesn't have a windowing system etc. As a result, to run the test
locally use `cargo test -- --ignored`. We should try to ensure we run
this at least once before publishing each major release.

Specifically, this test was created to help with testing for vulkano
image layout bugs (#271). cc @freesig just pinging you as you might
find this useful for this reason too!

MindBuffer/nannou