Add FindBugs profile to check for Hadoop incompatibilities.

I wrote a FindBugs plugin (https://github.com/tomwhite/hadoop-incompatibility-findbugs-detector) that checks for usage of Hadoop classes that have changed from classes in Hadoop 1 to interfaces in Hadoop 2. This PR uses the plugin to detect if Parquet has any incompatible calls.

I tested it by making a direct call on JobContext in ParquetInputFormat and it fails the build as follows:

```bash
$ mvn verify -DskipTests -Pfind-hadoop-incompatibilities
...
[INFO] --- findbugs-maven-plugin:2.5.2:check (default) @ parquet-hadoop ---
[INFO] BugInstance size is 1
[INFO] Error size is 0
[INFO] Total bugs: 1
[INFO] Found method invocation on incompatible Hadoop class org.apache.hadoop.mapreduce.JobContext
     in parquet.hadoop.ParquetInputFormat.getFooters(JobContext). ["parquet.hadoop.ParquetInputFormat"] At ParquetInputFormat.java:[lines 63-281]
```

It's in a profile so that it doesn't slow down the normal build cycle. Also, the plugin JAR is checked in since it's not in a Maven repo (yet).