Optimize battery life

[nelsonz]

Contingent on #24 and #25.

Most likely, we can use a 500-850mAh battery custom-made to the right size/thickness. With intermittent instead of constant reads (only possibly with multi-tagging), the power consumption could be low enough to last an entire day.

Possible problems: Safety. if we toss these in people's clothes, we need to make sure it's absolutely watertight and there is no possibility of shorts or busted lithium batteries. Nobody wants one of those going off near their crotch.

We should also add the receiver circuitry for wireless charging, since it adds little cost or volume and we can attempt the transmitter later on.