ACCUMULO-3450 Push audit configuration into Log4jConfiguration.

Ensure that the audit configuration is reset when the logging
subsystem is reset.
Merge branch '1.6'
ACCUMULO-3453 Bump commons-io dependency from 2.1 to 2.4.

Hadoop-2.7 is failing to build because this old version of
commons-io doesn't contain the necessary classes Hadoop
needs to run.
ACCUMULO-2815 Initial support for Kerberos client authentication.

Leverage SASL transport provided by Thrift which can speak GSSAPI,
which Kerberos implements. Introduced...

* An Accumulo KerberosToken which is an AuthenticationToken to
    validate users.
* Custom thrift processor and invocation handler to ensure server
    RPCs have a valid KRB identity and Accumulo authentication.
* A KerberosAuthenticator which extends ZKAuthenticator to support
    kerberos identities seamlessly.
* New ClientConf variables to use SASL transport and pass KRB
    server principal
* Updated ClientOpts and Shell opts to transparently use a
    KerberosToken when SASL is enabled (no extra client work).
* Ensure existing unit tests still function.
* Throw ThriftSecurityExceptions on bad authentication to ensure
    proper client action is taken.
* Fall back to krb principal before local OS user
ACCUMULO-2815 Unit test coverage for kerberos-related changes.