Change interfaces easier-to-use

I changed initializing flow of IssueReporter.

* Initialize IssueReporter on Application#onCreate called
* Enable to set mail body

```java
@Override
public void onCreate() {
    super.onCreate();
    ReportMail reportMail = new ReportMail.Builder()
            .email("support@example.com")
            .subject("Report an issue")
            .body(new SystemProfile(this).toString())
            .build();
    IssueReporter.start(this, reportMail);
}

@Override
public void onTerminate() {
    IssueReporter.stop(this);
    super.onTerminate();
}
```
### Links

* Abandon Android 2.x · Issue #2 · cookpad/issue-reporter-android https://github.com/cookpad/issue-reporter-android/issues/2