Fixes for MSVC

Fixes and improvements for Windows/MSVC, tested with msvc 2012/Windows 7 and gcc 4.7.2/Fedora 17.

* Pass the strutil and filesystem unit tests.
* Correct default font for ```oiiotool --text```.
* Enable the two digit exponent format earlier in oiiotool, allowing more plugin tests to pass.
* Fix some down-casting warning fixes.
* Added the build option ```CMAKE_USE_FOLDERS``` to organize the solution projects in folders. 
 
    ![oiio_folders](https://f.cloud.github.com/assets/601210/720080/b4e6d20e-dfab-11e2-8ca4-c27aab3e8180.png)