Move the example to top-level examples/ directory

That's where Cargo expects them to be.
.gitignore Cargo.lock

We're a library so we're supposed to ignore it.
Move from [[lib]] to [lib] in Cargo.toml

[[lib]] is deprecated now.
Enable Travis-CI

We don't have any tests and building the examples requires linking to
libtcod.{so,dylib,dll} so this just runs `cargo build` for now.