2.4.0

• New installation instructions, please review them if you are using Firebase: an additional step is required in this case. Closes #75, #80.
• Bump minSdkVersion to 16.
• Catch exceptions thrown by EventListener so we don't crash while a crash is occurring. Closes #71.
• Hide the app build date if it's before 1990. Since recent Gradle versions do not set this attribute anymore, I'm considering removing it in future releases.
• New CustomCrashDataCollector interface that allows specifying a class that will be invoked when a crash occurs in order to get additional data to be shown in the "error details" screen. Also added a new method CustomActivityOnCrash.getCustomCrashDataFromIntent(Intent) to retrieve the custom crash data reported by the CustomCrashDataCollector. Thanks @akallabeth for PR #77.