Bug 874462 - Add "startsWith" filterOp. r=reuben

The "contains" filterOp in ContactsAPI currently implements a filter that
only matches the start of each value when filtering by fields other than
"tel". This behavior is desirable for some uses, but does not match the
"contains" keyword.

Therefore, add a "startsWith" filterOp in addition to the current "contains".
A warning is sent to console.warn() if "contains" is used for non-"tel"
operations. A separate bug will be written to fix "contains" to perform full
text search.