Add possibility to find value according to a property of the model

I needed that feature to preload existing model objects that are not in the source list.
The comparison is made upon the property setted via the "track by" of the repeater.
Update select.spec.js
Revert "Merge branch 'fix-multiselect-population' of https://github.com/driver-by/ui-select"

This reverts commit 383f63bae832860a3830370a314399bb5098037a, reversing
changes made to 93434e13c99f8547c4acd389c119a3b3e12f8fab.
Merge branch 'multiselect'
bump version with PR256 merged