Add CurieIMUClass::end()

[eriknyquist]

Add end() method to IMU class, to disable clocking to the SPI
controller. This is unlikely to have noticeable power-saving effects,
however it is a good habit to get into.

[eriknyquist]

@bigdinotech @SidLeung @calvinatintel ready for review

[bigdinotech]

+1

[ndgbuilder]

I've detected that your changes do not match our desired coding style.

I created a patch to fix this, which you can easily download and apply to your
Pull Request using the following commands (Where is your locally
cloned fork):

cd <clone>
git checkout imuend
curl https://gist.github.com/ndgbuilder/f767050eb42e47f8ab0ceaf077fb616a/raw -o corelibs-arduino101-pr-235.patch
patch -p1 < corelibs-arduino101-pr-235.patch
git commit -a --amend --no-edit
git push origin imuend --force

If you want to avoid getting this message in future, please see this wiki page
for instructions on how to set up a pre-commit hook that will automatically
format your code before you push.

This is an auto-generated message

[eriknyquist]

@calvinatintel go ahead and merge

[calvinatintel]

cherry-picked