goimports
fix ActivityWorkerClient interface
add DescribeStream to KinesisClient interface
WorkflowClient -> WorkflowWorkerClient
add combined WorkflowClient interface
standalone constructor funcs for Pollers
use minimum required interfaces

Rather than using the actual Client implementation, use already-defined
client interfaces to make it clear which functions are needed, and to
make it easier to test and stub out alternate WorkflowClient
implementations.
log actual status code, make error check idiomatic
golint: don't use all-caps constant names
golint: Http -> HTTP, Json -> JSON, Id -> ID

Idiomatic Go uses consistent case for initialisms and acronyms
http://golang.org/s/comments
golint: use simpler Go idioms
golint: fix some comments and add missing ones
merge bgentry/interface-refactor