Dependency scopes and optionality

[dalaro]

@aadeon opened thinkaurelius/titan#741, but it looks like he meant to open it here.

I think he wants thrift-server to depend on slf4j-log4j12 with

<scope>runtime</scope>
<optional>true</optional>

Titan's top-level pom does this, though it's only proven marginally useful in practice. Some Titan modules need Log4j just to link, even if Log4j is disabled. HBase is one. I think Cassandra still has some log4j imports and nonreflective usage in StorageService and CassandraDaemon as of 2.0.x, though I guess logback is coming in 2.1 with https://issues.apache.org/jira/browse/CASSANDRA-5883.

Might also be nice to make junit <scope>test</scope>.

Anyway, your call. I can make a PR if you want.