Made the JS generator into a plugin that builds into a binary.

[haberman]

There is still a bit more work to do before gulp dist will work:

$ gulp dist
[16:50:45] Using gulpfile ~/code/protobuf-javascript/gulpfile.js
[16:50:45] Starting 'dist'...
[16:50:45] Starting 'genproto_wellknowntypes'...

../src: warning: directory does not exist.
Could not make proto path relative: ../src/google/protobuf/any.proto: No such file or directory

[16:50:45] 'genproto_wellknowntypes' errored after 19 ms
[16:50:45] Error: Command failed: sh -c bazel-bin/external/com_google_protobuf/protoc --plugin=protoc-gen-js=bazel-bin/generator/protoc-gen-js --js_out=import_style=commonjs,binary:. -I ../src ../src/google/protobuf/any.proto ../src/google/protobuf/api.proto ../src/google/protobuf/compiler/plugin.proto ../src/google/protobuf/descriptor.proto ../src/google/protobuf/duration.proto ../src/google/protobuf/empty.proto ../src/google/protobuf/field_mask.proto ../src/google/protobuf/source_context.proto ../src/google/protobuf/struct.proto ../src/google/protobuf/timestamp.proto ../src/google/protobuf/type.proto ../src/google/protobuf/wrappers.proto
../src: warning: directory does not exist.
Could not make proto path relative: ../src/google/protobuf/any.proto: No such file or directory