Added lib/commons-jexl-2.1.1.jar to .classpath; allow viewer.external.classpaths to include directories

2 changes:
- Added lib/commons-jexl-2.1.1.jar to .classpath (needed for compiling src/java/azkaban/trigger/Condition.java)
- Modified the viewer plugin external lib loading in AzkabanWebServer.java to allow viewer.external.classpaths to include directories in addition to files