Added methods for file to URL conversions to FileLocatorUtils.

When locating files it is often necessary to convert a File object to a URL.
The new methods simplify this task by already handling exceptions and returning
null in this case. This is the result returned by location strategies if a file
could not be resolved.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1525397 13f79535-47bb-0310-9956-ffa450edef68