[SPARK-1824] Remove <master> from Python examples

A recent PR (#552) fixed this for all Scala / Java examples. We need to do it for python too.

Note that this blocks on #799, which makes `bin/pyspark` go through Spark submit. With only the changes in this PR, the only way to run these examples is through Spark submit. Once #799 goes in, you can use `bin/pyspark` to run them too. For example,

```
bin/pyspark examples/src/main/python/pi.py 100 --master local-cluster[4,1,512]
```

Author: Andrew Or <andrewor14@gmail.com>

Closes #802 from andrewor14/python-examples and squashes the following commits:

cf50b9f [Andrew Or] De-indent python comments (minor)
50f80b1 [Andrew Or] Remove pyFiles from SparkContext construction
c362f69 [Andrew Or] Update docs to use spark-submit for python applications
7072c6a [Andrew Or] Merge branch 'master' of github.com:apache/spark into python-examples
427a5f0 [Andrew Or] Update docs
d32072c [Andrew Or] Remove <master> from examples + update usages
(cherry picked from commit cf6cbe9f76c3b322a968c836d039fc5b70d4ce43)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>