#133 Allow private fields as properties

[ljacqu]

• Use Field#setAccessible if a field is not private to make the code compatible with Kotlin constants without the need of the JvmField annotation.

[coveralls]

coverage: 99.39% (-0.2%) from 99.591% when pulling 89a2da44cbd4675d3c1cb5760c3a4b666f8f4941 on 133-support-private-settings-holder-fields into 168a12d8a1449bf60c7dcdce8371b3aa16f70676 on master.