.end: wait for server to close before invoking callback

If a server was spawned, wait for it to close before invoking .end's
callback. This allows the callback to launch another request against the
same supertest object without receiving a connection error.

Fixes #126.
Style nitpick
Add global leak checking
Fix global leaks
Merge branch 'benesch-master'
Pin devDependencies to exact versions
Release 0.13.0