Fix FileUtils#urlToFile() on Windows

Under certain circumstances, on Windows, URLs are translated into
strings of the form "file:C:/..." (which the URL constructor does not
like at all) instead of "file:/C:/..." (which the URL constructor
does like).

Let's be lenient on Windows and just accept even the first form.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a regression test for urlToFile(directory)

Java on Windows really has problems in that department: the default
AppClassLoader is a URLClassLoader whose getURLs() method returns URLs
of the form:

	file:C:/path/to/file.jar

i.e. omitting a slash: it should read

	file:/C:/path/to/file.jar

instead, otherwise, when converted into a URI via the toURI() method,
then passed onto a File constructor, one will see the lovely message:

	IllegalArgumentException: URI is not hierarchical

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add utility class for dealing with platform issues

In particular, it has some classic OS detection methods.
Accelerator: strengthen TODO to FIXME
Use PlatformUtils methods where applicable
Merge pull request #88 from scijava/urltofile-windows

Fix FileUtils#urlToFile() on Windows