Port to Windows 7/Visual Studio 2013

Apologies for the size of this pull request but it seems hard to split it in a meaningful way.

In porting the benchmark library I have tried to follow the model used by `glog`, and I have lifted some code from there.  Here is an overview of the changes:

1. Add a subdirectory `msvc/` containing the MS project and solution files.  There is one project for the library (`google-benchmark`) and one project for the tests (`google-benchmark-test`).  This stuff is just humongous blobs of automatically-generated XML, not much to review.
1. Change `.gitignore` and `.gitattributes` to properly handle the MSVC artifacts.
1. Add a `src/port.h` file (and the corresponding `src/port.cc`) to factor out the differences between Windows and the various Unix-ish OSes.  I lifted this file from `glog` and did the necessary adjustments.  In particular, it defines a bunch of `HAVE_` symbols that act as a poor man's `config.h`.  `src/port.h` must be included early in every source file that has OS dependencies, as among other things it defines the all-important symbols `OS_WINDOWS` and `COMPILER_MSVC`.
1. Change a bunch of includes to make them depend on the `HAVE_` symbols.
1. Implement `MyCPUUsage` for Windows.  No `ChildrenCPUUsage` yet.
1. Add a `pthread` library for Windows in `src/windows/pthread.{h,cc}`.  I got this library from http://locklessinc.com/articles/pthreads_on_windows/.  I did minimal changes to make it compile (mostly fixing casts) and I separated it into `.h` and `.cc` files instead of having everything in the `.h` file (which caused linker errors).  This library is distributed under a 3-clause BSD licence so I am assuming that using it is not problematic.
1. Other than porting, I made two significant changes to the code of the benchmark library:
  * In  `BenchmarkFamilies::RemoveBenchmark` I commented out some code which was changing a vector while iterating over it.
  * In `State::FinishInterval` I turned a `CHECK` into a warning because the wall time clock has poor accuracy on Windows 7.
@dominichamon, there is [`pthread win32`](https://sourceware.org/pthreads-win32/) - we can have a look at working out the correct [`ExternalProject_Add`](http://www.cmake.org/cmake/help/v3.0/module/ExternalProject.html) options to build that correctly on Windows. The `README` is pretty thorough for that project.