add a revision.txt to the runtime and helpers for querying

This allows for some runtime introspection:

```
$ bin/pyspark
In [1]: SparkContext.getRevision()
Out[1]: u'a349d974fec7f9d50de67056424d08bf8ec82dcb'
```

```
$ bin/spark-shell
scala> Version.getRevision()
res1: String = a349d974fec7f9d50de67056424d08bf8ec82dcb
```

this would put the current revision into `core/src/main/resources/revision.txt`

i hope that this works on PackServ. i could not test it locally because the build-script always removed my spark directory...

java resource loading is kind of crazy, if anyone has a better solution, i'm glad to change that.

/cc @Shopify/data-engineers