Make fromRect Optional (and other improvements)

This is a usability update with three significant changes:

When no fromRect prop is passed in, the popover scales from the middle of the screen, allowing for more use cases. This is very useful when the button that launches the popover is in a scrollview and may be in different positions, and thus calculating a rectangle would be difficult.
The content view is given a 10px padding from the edges of the screen in all cases. This makes it more visually evident that the popover is floating in front of the screen.
The entire view is now in a modal so that the popover always shows on top (This makes PR #27 not needed, which is better because zIndex is currently unreliable on Android).
Tapping on the content view itself no longer dismisses the popover, only tapping on the background view does. This allows for the use of a scrollview and other types of views in the popover that was not previously possible.

I have found these improvements make this component usable and useful on my iPad. Note: I do not have an Android tablet to test on, so this would be necessary before merging.

jeanregisser / react-native-popover

Make fromRect Optional (and other improvements) #30