Adjust initialization behavior

[njm]

This makes two changes to initialization for the integration:

• window._kmq is not clobbered if it already exists, e.g. on account of another tracking snippet; and
• assumesPageview is dropped to support tracking of events or properties before or without a page call.

[hankim813]

The assumePageview is generally used when partner libraries internally tracks a page call upon its own initialization. We do this so that we don't double count pageviews for integrations that behave that way. But looking at this, it looks like we're calling self.trackPage() in the callback of the load function... which kind of defeats the purpose of the assumePageview :/

Anyways, this lgtm! We're going through some analytics.js migration this week so we won't be able to deploy this until sometime next week.

Thanks for submitting the PRs! @njm