Support running totals using Mixpanels increment

Mixpanel's documentation on the increment call states that:
> For each key and value in the properties argument, adds that amount to the associated property in the People Analytics profile with the given distinct id.

For example, say I have a property called "Credits used" and its value is 5. If I make another increment with a value of 2 the resulting value of "Credits Used" should be 7.

This change allows increment totals to be made if the events properties contain a value.