Minor test cleanup
streams/changed: Fix arbitrary fns.

The arity detection patch only worked for keywords, not arbitrary
functions.
streams/changed: Emit pairs instead, with the :pairs? option.

Realized that the dual-arity function made streams/changed difficult to
compose with other primitives like smap; backing off to emitting
vectors.
Require riemann.streams, folds, test/io, test/tap in tests.
Moved test funcs from stream_test.clj to test.clj
run-stream now advances time.
Added stackdriver (https://www.stackdriver.com) plugin
Removed unnecessary variable
Used replace instead of split,join
Changed variable name which was creating confusion about data-point
whitespace
Updated stackdriver.clj
Added riemann.time as requirement
Updated stackdriver.clj to accept vector of events
Updated stackdriver adapter
Updated stackdriver to omit events with nil as the metric value
Added docstring
Merge pull request #410 from pradeepchhetri/master

Added stackdriver plugin.
streams/where-test: Expand to fully qualified tagged-any/tagged-all.

Fixes #414, where using `where` without streams/tagged in the local
namespace would fail to compile, or cause silent incorrect behavior.
Merge branch 'master' of github.com:aphyr/riemann