Actions API :: Cancel

Cancel method on the Actions API
Also a little bit of refactoring to move Actions related tests and code
into appropriate files.

http://reviews.vapour.ws/r/171/

Fix constants per review comments

use converter function
Merge pull request #910 from johnweldon/api-impl-cancel

Actions API :: Cancel

Cancel method on the Actions API
Also a little bit of refactoring to move Actions related tests and code into appropriate files.
Add apiserver/actions ServiceCharmActions endpoint

Also adds supporting types in apiserver/params/params.go and
apiserver/params/actions.go.
Add apiserver/actions ServiceCharmActions endpoint

Applied naming changes per @TheMue.
Merge pull request #909 from binary132/apiserver-servicecharmactions

Add apiserver/actions ServiceCharmActions endpoint

Also adds supporting types in apiserver/params/params.go and
apiserver/params/actions.go.