Remove assertion of type on batch context.

Was breaking compatibility with other batch engines, such as that
provided by drush (drush uses an object for context).
Merge pull request #3 from adam-vessey/7.x-remove-batch-context-type

Remove assertion of type on batch context.
Fix use of context 'results' in drush batch engine.

Was always replacing the results, and so was failing to accumulate
all the pages.
Merge pull request #4 from adam-vessey/7.x-context-results

Fix use of context 'results' in drush batch engine.