Upstream a number of changes to git.

Changes:
- Updates to README / os_setup for common installation issues
  by ebrevdo, vrv, wicke, anelia, dga

- Add a test for backwards compatibility checks by Josh

- Simplifications to tiling/padding of scalars by bsteiner

- Improve listing of devices when no devices are known by Matthieu

Base CL: 107607137

Author: Vijay Vasudevan

README.md

@@ -31,7 +31,9 @@ installing from source, GPU-enabled support, etc., see
 
 ## Binary Installation
 
-The TensorFlow Python API requires Python 2.7.
+The TensorFlow Python API currently requires Python 2.7: we are
+[working](https://github.com/tensorflow/tensorflow/issues/1) on adding support
+for Python 3.0.
 
 The simplest way to install TensorFlow is using
 [pip](https://pypi.python.org/pypi/pip) for both Linux and Mac.
@@ -40,7 +42,7 @@ For the GPU-enabled version, or if you encounter installation errors, or for
 more detailed installation instructions, see
 [here](tensorflow/g3doc/get_started/os_setup.md#detailed_install).
 
-### Ubuntu/Linux
+### Ubuntu/Linux 64-bit
 
 ```bash
 # For CPU-only version

tensorflow/core/common_runtime/local_session.cc

@@ -103,7 +103,11 @@ LocalSession::LocalSession(const SessionOptions& options,
   int devices_added = 0;
   if (options.config.log_device_placement()) {
     const string mapping_str = device_mgr_->DeviceMappingString();
-    printf("Device mapping:\n%s", mapping_str.c_str());
+    if (mapping_str.empty()) {
+      printf("Device mapping: no known devices.\n");
+    } else {
+      printf("Device mapping:\n%s", mapping_str.c_str());
+    }
     LOG(INFO) << "Device mapping:\n" << mapping_str;
   }
   for (auto d : device_mgr_->ListDevices()) {