[SPARK-6066] Make event log format easier to parse

Some users have reported difficulty in parsing the new event log format. Since we embed the metadata in the beginning of the file, when we compress the event log we need to skip the metadata because we need that information to parse the log later. This means we'll end up with a partially compressed file if event logging compression is turned on. The old format looks like:
```
sparkVersion = 1.3.0
compressionCodec = org.apache.spark.io.LZFCompressionCodec
=== LOG_HEADER_END ===
// actual events, could be compressed bytes
```
The new format in this patch puts the compression codec in the log file name instead. It also removes the metadata header altogether along with the Spark version, which was not needed. The new file name looks something like:
```
app_without_compression
app_123.lzf
app_456.snappy
```

I tested this with and without compression, using different compression codecs and event logging directories. I verified that both the `Master` and the `HistoryServer` can render both compressed and uncompressed logs as before.

Author: Andrew Or <andrew@databricks.com>

Closes #4821 from andrewor14/event-log-format and squashes the following commits:

8511141 [Andrew Or] Fix test
654883d [Andrew Or] Add back metadata with Spark version
7f537cd [Andrew Or] Address review feedback
7d6aa61 [Andrew Or] Make codec an extension
59abee9 [Andrew Or] Merge branch 'master' of github.com:apache/spark into event-log-format
27c9a6c [Andrew Or] Address review feedback
519e51a [Andrew Or] Address review feedback
ef69276 [Andrew Or] Merge branch 'master' of github.com:apache/spark into event-log-format
88a091d [Andrew Or] Add tests for new format and file name
f32d8d2 [Andrew Or] Fix tests
8db5a06 [Andrew Or] Embed metadata in the event log file name instead