underiveQualifiedKey doesn't reverse deriveQualifiedKey

[spinnytea]

deriveQualifiedKey has a particular implementation: 'prefix' + 'key'
underiveQualifiedKey should reverse the process using the same logic (i.e. key.substr(prefix.length))

while building a regex to replace the beginning is fun, it doesn't work if the prefix is itself a regex.

For example, if the prefix is '...', then it will match anything in local storage that's at least three characters long. Sure it will basically do the right thing if angular-local-storage is the only thing using local storage for the site.

Another example, if the prefix is 'my[wonderful]site', then it won't ever do the right thing.

It looks like that function is only used in one place. And there's one other place that builds a regex to test for the prefix. All the other places use the string functions for stripping the prefix (instead of using this function).

Great tool! It's super easy to use.