Typo in openImageInWindow
Use chrome.storage.local instead of storing through background page
Add option to limit cache usage.

User can change maximum cache size in options. The default value is
chrome.storage.local.QUOTA_BYTES. If the quota is exceeded or the
maximum cache size value, some random cache values are deleted.
Cannot tell which ones are the oldest or which ones are more
important. storing further information in cache would limit it
capacity, therefore just delete some random value.

Hz.Cache add functions to:
* remove data untill it fits into the user defined size
* query the used size

Hz namespace change:
* Init in common.js, other files just add to it.
* Reduces dependencies between files