TxEddystone, initial cut.

[sewelljustin]

Implements a tabbed layout of 4 BLE advertisers, each capable of
broadcasting an Eddystone UID, TLM or URL frame, each with independent
Tx power and mode.

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[scottjenson]

Have you had a chance to fill out the CLA?

[mashbridge]

Hi Sewell. This looks like it's mostly my https://github.com/google/eddystone/tree/txeddystone/tools/txeddystone/TxEddystone which I haven't yet merged into master. (Needs a touch more testing which I haven't got around to yet.) Could you describe the differences between your PR and that branch?