RFC: Add windows support for shared memory and SharedArray.

 - Added cross-platform (POSIX and windows) interface for shared memory (in `base/shmem.jl`).
 - Edited `SharedArray` to use this new interface.
 - Edited the test scripts and documentation to reflect the new support for windows.
 - Verified `test/parallel.jl` runs successfully on 32-bit and 64-bit windows and 64-bit linux.
We should retain the informational printing of shared memory limits on an error.  Will help folks debug errors due to system limits faster.