Crashes if the browser doesn't support localStorage

[peter lee]

Crashes if a model is using localStorage and browser doesn't support localStorage.

// from Safari private browsing
QuotaExceededError: DOM Exception 22: An attempt was made to add something to storage that exceeded the quota.

Should extend Spine.Model.Local only If localStorage is supported.

Such line(

spine/src/local.coffee

Lines 4 to 6 in 6a023ab

 extended: -> 
   @change @saveLocal 
   @fetch @loadLocal 

) should check if the browser supports localStorage:

 extended: ->
    if window.localStorage
        @change @saveLocal
        @fetch @loadLocal

[Richard Flosi]

👍