Maven dependencies fix

[alexismeneses]

All third-party maven dependencies are now optional. Ones relying on features that need optional dependencies should declare it in their own pom as they're no longer transitive.

This is particularly true for users of SSPI functionality which requires waffle on the classpath. Following lines should be used in that case:

<dependency>
  <groupId>com.github.dblock.waffle</groupId>
  <artifactId>waffle-jna</artifactId>
  <version>1.7</version>
  <scope>compile</scope>
</dependency>

No concrete implementation of slf4j is made transitive anymore (fixes #251)