Support :if and :unless for track_history callbacks

[jagdeepsingh]

Raised for #209 with RSpecs here.

[coveralls]

Coverage decreased (-1.0%) to 98.806% when pulling 647f606 on JagdeepSingh:conditional-if-unless-options into d6102a1 on mongoid:master.

[coveralls]

Coverage increased (+0.007%) to 99.767% when pulling 647f606 on JagdeepSingh:conditional-if-unless-options into d6102a1 on mongoid:master.

[coveralls]

Coverage increased (+0.007%) to 99.767% when pulling 647f606 on JagdeepSingh:conditional-if-unless-options into d6102a1 on mongoid:master.

[coveralls]

Coverage increased (+0.007%) to 99.767% when pulling 647f606 on JagdeepSingh:conditional-if-unless-options into d6102a1 on mongoid:master.

[johnnyshields]

@dblock I'd appreciate if you can take a moment to merge 212, 211, and 210 and ideally cut a release. Both Jagdeep and myself have reviewed these.

[dblock]

Done. And this looks great, rebase/merge along.

[johnnyshields]

@jagdeepsingh please handle

[jagdeepsingh]

This will be conflicted again after merging #211. Will rebase it then.

[coveralls]

Coverage increased (+0.006%) to 99.803% when pulling dcdf689 on JagdeepSingh:conditional-if-unless-options into f9880c7 on mongoid:master.

[jagdeepsingh]

Rebased to latest master. @dblock Please merge.

[dblock]

Great, merged.

[johnnyshields]

@dblock thanks for all your help. Can I ask you to cut an official release?

[dblock]

I can try to get to it in the next day or so, otherwise you can cut one too :)