FLUME-2502. Improve Spool Directory Source's performance by not listing files each time.

(Prateek Rungta via Hari)
FLUME-2499. Include Kafka Message Key in Event Headers.

(Ricky Saltzer via Hari)
FLUME-2486. TestExecSource fails on some environments

(Santiago M. Mola via Hari)
[FLUME-2215] ResettableFileInputStream can't support ucs-4 character.

Patch by Alexandre Dutra:
https://issues.apache.org/jira/secure/attachment/12620855/FLUME-2215-1.patch
Ensure proper file permissions on TestHDFSEventSinkOnMiniCluster.

MiniDFSCluster will fail if DFS_DIR has permissions other than
rwxr-x-r-x. This is usually accomplished by having umask set to
0022, which is unconvenient for testing. So we try to set such
permissions with setfacl.
Add Travis CI conf.
Fix TestResettableInputStreamReader.
Adapt TestResettableInputStreamReader to Java 8.
Fix TestLog.doTestMinimumRequiredSpaceTooSmallForPut.

Travis CI had problems executing this test as is:
https://travis-ci.org/Stratio/flume/jobs/36758319#L6178
Update apache parent pom to version 15.
travis: Raise ulimit for new threads.

Solves Travis CI exception on oraclejdk7:
https://travis-ci.org/Stratio/flume/jobs/36814709#L5747
https://github.com/travis-ci/travis-ci/issues/2496
[FLUME-2203] Add jsr-305 dependency.
[FLUME-2286] Upgrade Guava to 17.0.
Upgrade snappy-java to 1.1.1.3.
travis: Make hostname point to 127.0.0.1. Fixes HBase tests.