Insecure 'close' image over HTTPS

[chaegribben]

The "close.src" image location is hard-coded into this with an HTTP:// which causes warnings when the plugin is used through HTTPS.

On line 228 of infobubble.js the "http:" should be removed, leaving the link as:

//maps.gstatic.com/intl/en_us/mapfiles/iw_close.gif

This will then work over HTTPS without issue. It would also be beneficial to add an attribute for 'closeSrc' so that the close button image source can be specified by the developer.

[chaegribben]

For reference see the same issue on the original codebase here:
https://code.google.com/p/google-maps-utility-library-v3/issues/detail?can=2&start=0&num=100&q=&colspec=ID%20Type%20Status%20Priority%20Fixed%20Owner%20Summary%20Stars&groupby=&sort=&id=184