Use object property of track by for comparison

[richardallen]

This expands on PR #256 to improve efficiency, clean up the code, and use track by in single as well as multiple configurations.

• Moved the regular expression that determines the property name into RepeatParser.
• Cleaned up the formatter logic by breaking into functions that are created only once when the formatter is added.
• Used the object property specified via track by for comparisons in both single and multiple configuration.
• Added unit tests for both single and multiple configuration.
• Used angular.equals instead of == for comparison. The AngularJS ngOptions directive notes that it compares by reference, not value, so it seems natural that angular-ui-select would do the same. However, several developers have posted issues on this behavior causing problems, so using angular.equals seems like a better choice. This change was made with a separate commit, so the rest of this PR could be merged without that commit.

[ilanbiala]

@richardallen some changes made, you probably need to fix up this PR.

[user378230]

Closing given that codebase has changed significantly since this PR was created and many of the changes/fixes probably no longer apply.