Only build carthage scheme for CI by default

It encompasses CarthageKit and its tests already, so no need to build
everything.
Add .travis.yml
Work around Xcode beta Optional incompatibility
Notify Slack, not email
No travis emails, please
Regenerate Slack key with encryption
Refactor to use dispatch_io for output aggregation

And wait for EOF before forwarding it to the ColdSignal subscriber.
Always flush Xcode log file after writing
Upcast dispatch_data_t to NSData
Merge remote-tracking branch 'origin/master' into fix-task-output-race
++RAC to hot-signal-lifetime for development
Stop prematurely disposing of output signals

Because we would dispose of build output subscriptions when the _signal
of signals_ was disposed (not the inner, flattened stream), it would
happen too early, resulting in no build output at all.

Instead, now that RAC is fixed, we can rely upon the lifetime of the
captured sinks we’re writing to. When the sink leaves scope, output
signals (and therefore observations) will automatically disappear too.

Fixes #151.
++RAC to improve-synchronous-cold-signals
Refactor users of ColdSignal for new API
Merge branch 'master' into travis-ci
Add KEY_PASSWORD envvar for codesigning
Add encrypted iOS development certificate
Create a temporary keychain on Travis for signing
Verbosify
Fix verbose `security` invocations