Tidy `proxy.js`.

Removed useless return statements.
Tidy.

Wrapped long lines. Checked spelling. Moved commas to end of line.
Upgrade Proof to 0.0.20.
Merge branch `master` into `client-test`.
Minimal test through proxy.

Created a minimal test runs a UDT connection through a proxy. The
connection is between a C++ client and a C++ server to verify that the
proxy itself works correctly.
Tidy `.gitignore`.
Add Proof errors to NPM test build.
Adjust tests for Travis CI build.

Adjusted `t/sdk/proxy.t.js` for Linux. On Darwin the server process
exits successful, while on Linux the exit is null and the signal is
reported.

Updated the `stdout` listener to fire only once. It was firing twice on
Travis CI, causing the test to setup a second proxy which failed because
the ports were already bound.
Merge branch `proxy-test` into `master`.