Use container width instead of device width

[lukescott]

Currently the device width is used to calculate the width of the calendar, and then each day is /7 of that width. This is a problem on tablets that have large screens. You more than likely will want to reduce the size of the calendar to be less than the width of the screen, especially in landscape.

The following patch uses onLayout to figure out the width of the parent container and uses that instead of the device width. It then uses that /7 to for the day width. I had used flex:1 for the day width, but I also wanted the height to match the width - that way each day doesn't end up being a narrow rectangle depending on the container width.

I also fixed a bug where it would always select the current day, even though this.props.selectedDate is undefined. This happened because moment(undefined) returns the current date.

This patch also makes it easier to do 2 months side-by-side down the road. To do that you would likely: divide containerWidth by 2, update getMonthStack to render more months, and change the scroll functions to take 2 months into account.

[christopherdro]

Closing for #107

[coderdave]

I bit the bullet and resolved the conflicts on this one. It works pretty well going back and forth between portrait and landscape, so that's good. The only thing that didn't make sense to me was the width/height that was added to the Day component. I took that out because it caused the days to be a lot taller than normal.

I encourage anyone (especially @christopherdro) to check out the webconnex-container-width branch and if everything is fine, I can merge it to dev.