feat(omaha): add disablebackoff field to action

another extension added by the the chromeos update engine
fix(omaha): create section for update_engine extensions

add this comment section just so it is a littlemore clear what is going
on.
feat(omaha): add more update_engine extensions
Merge pull request #4 from philips/update-engine-extensions

Update engine extensions
terminated XMLName from appearing in json
when we need json encoding(putting AppRequestLogs into bigquery)
we don't want the XMLName tag adding on needless data
Merge pull request #5 from doodles526/working

terminated XMLName from appearing in json
added json omitempty tags
bigquery was throwing errors without these due to null defined fields
Merge pull request #6 from doodles526/working

added json omitempty tags
feat(omaha): add Track field

this is another update_engine extension. Add it.
chore(omaha): group the update engine extensions together
Merge pull request #7 from philips/add-track-extension

Add track extension
feat(omaha): add a bootid field

the bootid field will be used by CoreOS to uniquely identify a boot of
an instance.
README: add a link to the go docs
fix(omaha): go fmt
feat(omaha): add oem and previousbootid

Add two new extensions for coreos
Merge pull request #9 from philips/add-new-fields

philips/add-new-fields
fix(omaha): Oem to OEM

Be consistent with the Go style guide and make the acronym fully
uppercase.