Add Ivy configuration to download dependencies for project on demand

This change invokes Ivy as our dependency manager so that the appropriate jars
are automatically downloaded during CI builds on Travis (and local dev builds
too, for that matter). This simplifies testing and allows the app to be
packaged more easily.

Summary:

* Adds Travis CI build status image to README
* Updates Travis configuration to execute "init-ivy" ant task to download and
prep dependencies into lib directory
* Adds lib directory to ignored directories in .gitignore
* Fixes to Main.java to reference new HttpSolrServer class instead of
deprecated CommonsHttpSolrServer
* Always uses lib directory created by Ivy and removes now-unneeded
build.properties
* Makes Java "invoke" target fail the build if the app fails
* Makes "test" target the default since the "run" target does not yet run on
Travis since it requires a running Solr server