Added x-request-start header for each request

Signed-off-by: Phan Le <ple@pivotallabs.com>
prepare to fix cf-release gorouter packaging

Signed-off-by: Phan Le <ple@pivotallabs.com>
Go fmt
Refactor Proxy to be an interface with fewer dependencies

Signed-off-by: Phan Le <ple@pivotallabs.com>