Add the ability to remove toggle listeners that you have added

[StylianosGakis]

About the changes

This adds two new public functions to RefreshPolicy, and by extension to UnleashClientSpec so that the caller can call them.

Closes #68

Discussion points

I have not added remove functions for the other listeners, as they do not currently expose a function to add listeners through the UnleashClientSpec.
I have added a test to RefreshPolicyTest itself to ensure that adding and removing the listener works as expected.

The first commit fixes a little bug I think was there regarding what object was used for synchronization, which I think was simply a misspell. If that was not the case feel free to either revert that yourself or let me know so I can do it.

[chriswk]

LGTM

[StylianosGakis]

Thank you for merging this in!
As a follow up question here, what do you typically do regarding release cycles of the SDK? Is there a standard cycle that you make a new release in if there are changes to be released, do you do it when you got something bigger going, or what do you do in general?

I am trying to understand when I could expect this change to be included in the latest release, since I do not see anything related to snapshot releases or something like that which would allow me to use this before the next release.