Windows installation support

This PR adds stuff discussed in issue https://github.com/openlayers/ol3/issues/427.
Following elements were added:

* ```which()``` function that checks if exectuables exist.
* ```checkdeps``` option for build.py (thanks @marcjansen)
* a new ```sys.platform == 'win32'``` section.

The ```checkdeps``` option is really nice, because it enables a quick
check of all dependencies on Windows (which sometimes might be
really tricky).

I tested this PR by running:
* ```python build.py checkdeps``` -> OK
* ```python build.py serve``` -> OK
* ```python build.py test``` -> OK
* ```python build.py integration-test``` -> almost OK

The last test hangs up at line:
 ```2013-03-31 10:55:37,286 check-examples:
 phantomjs.exe bin/check-example.js 
build/gh-pages/master/examples/bind-input.html?mode=raw```

I don't think that this is because of these changes, but I can't get why it hangs up?
Moreover, I did let it run for over 30 minutes...

Limitations of this PR are:
* We assume that phantomjs version is ```1.9.0```, if not defined in ```Path```.
* We assume that Java jdk version is ```1.7.0_17```, if not defined in ```Path```.

I also created a specific wiki page on my fork: 
https://github.com/kalbermattenm/ol3/wiki/Windows-install-HOWTO