Making clustered slacker client lazy-init. Servers are pulling from zk
the first time client calls that remote function. defn-remote in
slacker-cluster will neither generate any remote calls.
added ping support in clustered client
fixed close function of normal client
added url in project
added :first to grouping strategy. the client will always call the
first server available.

resolved the sticky server feature request #2.
pmap on multiple server call by default
changed internal api for sync-all and inspect.
introduced new option :group-exceptions
async grouped promise. part of #6 fix.
code cleanup
updated async callback, only triggered when grouped call completed
update callback API as upstream changes
do not throw exception in ClusterEnabledSlackerClient, only returns
call-result map.
Fixed issue when (zk/children) returns false on empty node
Fixed #7. Totally rewrite unit tests. Removed zookeeper dependencies.
removed log4j from dependencies. leave it to user.
setup travis
fixed error in project.clj
bring back log4j as a compile dependency
Merge branch '0.11-dev'
fixed grouping-exception conditions
bump version and update README