Provide a base class for subcontexts

[pfrenssen]

It would be helpful if we provide a base class for subcontexts.

• Now all subcontexts have to implement the constructor but this is likely to be common for all subcontexts, causing code duplication.
• We can extend RawDrupalContext which would be a sane default for the most common case of subcontexts provided by contributed modules.
• If we have a base class we can also provide some helpful methods that can assist developers in building their subcontexts. As an example I have provided a method that allows the subcontext to get the currently logged in user from the DrupalContext class.

[jhedstrom]

This looks great!

Could you re-push this now that the module error has been fixed in the drupal driver?

(I wish there was a way to re-queue travis builds.)

[jhedstrom]

Oh, nevermind, Travis added a rebuild button!

[openprivacy]

So where I used to extend RawDrupalContext I should now extend DrupalSubContextBase as a general rule?

[jhedstrom]

If you're writing subcontexts you can extend this if you don't want to repeat all the boiler plate code it includes. For normal contexts, you can still extend RawDrupalContext.