i2c: Adds i2c.Addr type and flag.Value interface. (google#362)

* i2c: Adds i2c.Addr type and flag.Value interface.

Add i2c.Addr
Add i2c.Addr Set()
Add Tests.

mt7688: Fixes pin function naming (google#313)

ina219: add i2c driver for high side current and voltage sensor (goog…

…le#292)

bmxx80: fix humidity sensing

It regressed in 6423b54. :( This is due to
integer overflow. Tested against the executable compiled from periph v2.

Refactor the tests to make the error messages clearer.

Fixes google#256

display: add missing package doc

headers drivers: use After instead of Prerequisites

Now that the allwinner and bcm283x Pin can work even if the driver failed by
falling back into sysfs-gpio, the header should load even if the CPU GPIO
drivers failed.

allwinner-gpio-pl is now after sysfs-gpio instead of allwinner-gpio as a
prerequisite.

This will enable cases like running Arch on a RPi3 where gpiomem kernel driver
is not loaded, the process is not running as root, so /dev/mem cannot be memory
mapped, but sysfs-gpio loaded successfully.

.gohci.yml: add beaglebone-4373

Only include simple things for now.

benchmarks: add InNaive, fix Hz units display

sysfs: Improve coverage for thermal

- Correct minor typos.
- Improve sysfs-thermal error wrapping.
- No significant functional change.

Remove the warning in README.md about versioning.

Now that v1.0.0 is being released and will use a proper versioning mechanism,
there's no need for this warning.

Clients will be able to use http://github.com/golang/dep to version this
library.