Add build system for compiling under MSVC x64

Add build system for compiling under MSVC x64

This patch adds the modifications needed to compile under x64 under
Windows:
- created a new macro for testing if we are compiling under x64.
this will define the linker flag: "/MACHINE:X64" as per documentation
(https://msdn.microsoft.com/en-us/library/9yb4317s.aspx).

- added x64 pthread libraries under the pthread defines

- add documentation on how to build under x64

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>