Fixes execution guard bug preventing server start.

Since the release of neo4j-community-1.9.M01, a bug exists which causes the server to fail to start when the execution_guard_enabled flag is set to true in neo4j.properties as well as setting org.neo4j.server.webserver.limit.executiontime in neo4j-server.properties.  

Specific scenario to reproduce in 1.9.M01 - 1.9.M04

neo4j.properties
```
execution_guard_enabled=true
````

neo4j-server.properties
```
org.neo4j.server.webserver.limit.executiontime=1000
````

The specific exception thrown is:

```
java.lang.NullPointerException
	at org.neo4j.server.web.Jetty6WebServer.addExecutionLimitFilter(Jetty6WebServer.java:549)
	at org.neo4j.server.AbstractNeoServer.startWebServer(AbstractNeoServer.java:307)
	at org.neo4j.server.AbstractNeoServer.start(AbstractNeoServer.java:150)
	at org.neo4j.server.Bootstrapper.start(Bootstrapper.java:81)
	at org.neo4j.server.Bootstrapper.main(Bootstrapper.java:46)
2/4/13 3:24:29 PM org.neo4j.server.AbstractNeoServer SEVERE: Failed to start Neo Server on port [7474], reason [null]
```

This pull request detects the uninitialized server and throws a more informative exception.
It also adds a test to detect this bug and patches Jetty6WebServer to correct
the issue.