Refactored URLs

Signed-off-by: Phan Le <ple@pivotallabs.com>
Make organization_id required in the logMessage protobuffer

Signed-off-by: Phan Le <ple@pivotallabs.com>
Extracted LogTarget class to encapsulate org, space, app selection

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