Allow IonWriters to be reused.

[zslayton]

This PR addresses #189.

Changes:

• Bumps ion-java dependency from 1.4.0 to 1.5.1 (latest stable release)
• Adds boolean _ionWriterIsManaged field to the IonGenerator class, allowing it to track whether the provided IonWriter should be closed when the IonGenerator is closed.
• Modifies the IonFactory to specify whether the IonWriter it is providing to new IonGenerator instances was just created (ionWriterIsManaged=true) or if it was provided by the user (ionWriterIsManaged=false).
• Adds unit tests showing that the same instance of IonWriter can be reused for multiple calls to IonObjectMapper#writeValue(IonWriter, Object).