Enable golint & gofmt, resolve issues (#214)

* Add golint to linters and resolve issues.

I decided to remove constructors for some of the propagation types
because the constructors can be reduced to either using the zero value
or a single, non optional member.

* Enable gofmt and commit fixes

propagation/binary_propagator.go

@@ -27,7 +27,7 @@ var _ apipropagation.BinaryFormatPropagator = binaryPropagator{}
 
 // BinaryPropagator creates a new propagator. The propagator implements
 // ToBytes and FromBytes method to transform SpanContext to/from byte array.
-func BinaryPropagator() binaryPropagator {
+func BinaryPropagator() apipropagation.BinaryFormatPropagator {
 	return binaryPropagator{}
 }