Multi-line or multi-statement Python scripts raise NullPointerException

Calling a multi-statement score using curl e.g. `a=22; _score*a` is not possible with the current lang-python, nor is it possible to write a script using semicolons or multiple lines. I can only get compound statements (e.g. list comprehensions with embedded if statements) to work, so I'm limited in the complexity of my scoring process.

I'm using ES 1.3.2 and latest lang-python:
```
$ ls -la /usr/share/elasticsearch/plugins/lang-python/
-rw-r--r-- 1 root root    10482 Aug 27 17:20 elasticsearch-lang-python-2.3.0.jar
-rw-r--r-- 1 root root 14340135 Aug 27 17:20 jython-standalone-2.5.3.jar
```

Here's a worked example:

```
# Delete existing data, add 2 simple records, fetch both results to stdout
curl -XDELETE "http://localhost:9200/test"
curl -XPUT "http://localhost:9200/test/doc/1" -d '{
  "num": 1.0
}'
curl -XPUT "http://localhost:9200/test/doc/2?refresh" -d '{
  "num": 2.0
}'
# show our records
curl -XGET 'http://localhost:9200/test/_search' -d '{
   "query" : { "match_all": {}}
}'
```

We'll run a simple query that uses `num` as a score modifier:
```doc["num"].value * _score```

If I create `/etc/elasticsearch/scripts/py1.py`:
```
doc["num"].value * _score
```

and wait for the script to reload (by monitoring the logs), I can call:
```
$ curl -XGET "http://localhost:9200/test/_search?pretty" -d'
{
  "query": {
    "function_score": {
      "script_score": {
        "script": "py1",
        "lang": "python"
      }
    }
  }
}'
```
and this will calculate the results.

The same can be achieved using an in-line call (ignoring `py1.py`):
```
curl -XGET "http://localhost:9200/test/_search?pretty" -d'
{
  "query": {
    "function_score": {
      "script_score": {
        "script": "doc[\"num\"].value * _score",
        "lang": "python"
      }
    }
  }
}'
```

However using more than 1 statement will fail. This example uses `;` to split 2 statements (this is legal in jython 2.5):
```
curl -XGET "http://localhost:9200/test/_search?pretty" -d'
{
  "query": {
    "function_score": {
      "script_score": {
        "script": "a=42; doc[\"num\"].value * _score",
        "lang": "python"
      }
    }
  }
}'

      "reason" : "QueryPhaseExecutionException[[test][3]: query[function score (ConstantScore(*:*),function=script[a=42; doc[\"num\"].value * _score], params [null])],from[0],size[10]: Query Failed [Failed to execute main query]]; nested: NullPointerException; "
and the log message:
org.elasticsearch.search.query.QueryPhaseExecutionException: [test][3]: query[function score (ConstantScore(*:*),function=script[a=42; doc["num"].value * _score], params [null])],from[0],size[10]: Query Failed [Failed to execute main query]
	at org.elasticsearch.search.query.QueryPhase.execute(QueryPhase.java:162)
	at org.elasticsearch.search.SearchService.executeQueryPhase(SearchService.java:261)
	at org.elasticsearch.search.action.SearchServiceTransportAction$5.call(SearchServiceTransportAction.java:206)
	at org.elasticsearch.search.action.SearchServiceTransportAction$5.call(SearchServiceTransportAction.java:203)
	at org.elasticsearch.search.action.SearchServiceTransportAction$23.run(SearchServiceTransportAction.java:517)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
```

Creating a `py2.py` in the scripts directory containing:
```
a=42; doc["num"].value * _score
```
and calling
```
$ curl -XGET "http://localhost:9200/test/_search?pretty" -d'
{
  "query": {
    "function_score": {
      "script_score": {
        "script": "py2",
        "lang": "python"
      }
    }
  }
}'
has the same error:
      "reason" : "QueryPhaseExecutionException[[test][3]: query[function score (ConstantScore(*:*),function=script[py2], params [null])],from[0],size[10]: Query Failed [Failed to execute main query]]; nested: PyException; "
```

If a `py3.py` script is made with the same two statements split over two lines:
```
a=42
doc["num"].value * _score
```
then the same errors are thrown.

I'll note that if I experiment with equivalent groovy scripts then both the semicolon is allowed and multi-line scripts (in /scripts) are allowed.

Closes #19.