Allow to write to PathURLConnection

Hey,

we ran into some issues in our unit tests, where EMF is trying to write to some URL in the jimfs filesystem.

The stack is the following:
```java
PathURLConnection.getOutputStream() line: 130 => "protocol doesn't support output"
URIHandlerImpl.createOutputStream(URI, Map) line: 154
ExtensibleURIConverterImpl.createOutputStream(URI, Map) line: 349
PlatformResourceURIHandlerImpl.createOutputStream(URI, Map) line: 498
ExtensibleURIConverterImpl.createOutputStream(URI, Map) line: 349
ModelResourceImpl(ResourceImpl).save(Map) line: 1041
```
To make EMF aware of jimfs, we register the top-level URI of a jimfs path into the ecore 'platform resource map' (essentially a map from Eclipse IProject -> URI) in the test setup:

```java
EcorePlugin.getPlatformResourceMap().put("VdkWithVHub", URI.createURI(top.toUri().toString()));
```

It would be good, if the [PathURLConnection](https://github.com/google/jimfs/blob/master/jimfs/src/main/java/com/google/common/jimfs/PathURLConnection.java) could also implement the write protocol.

A trivial implementation fixed the issue for us: [pull request](https://github.com/google/jimfs/pull/160)

We would kindly ask to review the change and make suggestions if you see any violations of the protocol.

also see: #13 [Obtaining a URL from a jimfs path](https://github.com/google/jimfs/issues/13)

Thanks
Janik