Issue #12: Implement toProto and TransportHTTPProto

The Proto transport should be considered beta. No performance
testing has been done.

Known issues:
- GUIDs larger than PHP_MAX_INT will be truncted, this could lead
to invalidly joined traces and/or traces that fail to join

Makefile

@@ -58,4 +58,4 @@ thrift:
 proto:
 	protoc --proto_path "$(PWD)/../googleapis:$(PWD)/../lightstep-tracer-common/" \
 		--php_out="$(PWD)/lib/generated" \
-		collector.proto
+		collector.proto google/api/annotations.proto google/api/http.proto

composer.json

@@ -3,7 +3,8 @@
     "description": "LightStep instrumentation API",
     "license": "MIT",
     "require": {
-        "ruafozy/mersenne-twister": "^1.3"
+        "ruafozy/mersenne-twister": "^1.3",
+        "google/protobuf": ">=3.6.1"
     },
     "require-dev": {
         "phpdocumentor/phpdocumentor": "^2.8.5",