Fix flushes all Burry items test

Browsers do not respect the order of the keys as the tests require.

Test failed on Firefox 26, Chrome 32 on Ubuntu.
Refactor set()

Reduce code duplication in set()
Update quota exceeded detection method
Merge pull request #6 from duego/fix-flush-test

Fix flushes all Burry items test
Merge pull request #7 from duego/refactor-set

Refactor set()
Merge pull request #8 from duego/test-quota

Update quota exceeded detection method