Issue 125. changed Custom Fields to be a Set instead of List
to avoid "duplicate ID" exception when several custom fields
with the same ID are added to the list due to programmer's error.

Closed direct access to CustomFields and other collections in Issue, User and other beans.
User Unmodifiable Collections in Issue and User classes.

Added Fest Assert library to use assertThat() style assertions in tests.
Merge pull request #149 from taskadapter/ask-125-new-duplicate-ids

Issue 125. changed Custom Fields to be a Set instead of List