SONARFBUGS-7 Upgrade to findbugs 3.0.0

This forces to use Java 7 for execution of tests.

There is 4 new rules:
* NP_OPTIONAL_RETURN_NULL
* IIO_INEFFICIENT_INDEX_OF
* IIO_INEFFICIENT_LAST_INDEX_OF
* CNT_ROUGH_CONSTANT_VALUE
Merge pull request #2 from Godin/findbugs-3.0.0

SONARFBUGS-7 Upgrade to findbugs 3.0.0