adds a scoped object to store values under scope-wide names

[lukasheinrich]

the workflow view has new methods

• addValue(key, value)
• getValue(key)

note: keys are only write-once. They cannot be overwritten,
once they have a value assigned.

where the value being stored can be an expression that should
be executed against the Workflow Object. This will allow
stages use expressions stored under these 'well-known' names
inst. E.g.

instead of

parameters: {stages: foo, output: bar}

one could do
parameters: {expression_type: fromvalue, key: ascopewidekey}

assuming another stage has written e.g. the expression
'{stages: foo, output bar}' to 'ascopewidekey' the present
stage does not need to know anything about stage 'foo'

this will enable pattern such as taget-based workflows in which
dependencies are expressed via as scope-wide target names.

i.e. result data is accessible under unique "addresses"

• stagename/nodeindex/result
• nodeid/result
• valuekey

[coveralls]

Coverage decreased (-0.4%) to 86.712% when pulling 630d0fb on values into 347ccad on master.