Show empty IntervalCollection
Show empty IntervalCollection

Fixes the `show` method for an empty `IntervalCollection`. Before, 
    IntervalCollection{Int}()
failed with
    ERROR: `IntervalCollectionIteratorState{Int64}` has no method matching IntervalCollectionIteratorState{Int64}()
    ...

timbitz/Bio.jl,kdmurray91/Bio.jl,richardreeve/Bio.jl,JuliaPackageMirrors/Bio.jl