Initial work for clients to use context to manage request deadlines.
Pass context as first request parameter as suggested in the docs.

Url: http://godoc.org/code.google.com/p/go.net/context
Added context helper to execute actions that can be cancelled.
Request context helpers for tracking request correlation.
Bugfix for decoding response common errors.
Added a function for creating a context specific logger.
Req client is now thread safe for multiple requests.
Merge branch 'feature/service-client-context' into develop