A native port for Windows

Add a Visual Studio solution to build a native port of UDUnits2 on
Windows. Some POSIX specific routines are ported as well. The test suite
can be run from the test project and gives the following results :

Run Summary:    Type  Total    Ran Passed Failed Inactive
suites      1      1    n/a      0        0
tests     35     35     33      2        0
asserts    833    833    830      3      n/a

A quick look at the 3 failed shows that they are mainly related to
formating of numbers, e.g. 1.2e018 instead of 1.2e18 (mind the extra
zero).
Test config is Win7/64 with msvc 11 (aka 2012)

Build instructions are in the win subdirectory.

The implementation mainly consists in macro testing either _WIN32 or _MSC_VER
whichever is more relevant.