use agent to keep connections alive

fixes:
https://github.com/joyent/node/issues/6833
https://github.com/triAGENS/ArangoDB/issues/985
regard node version

config agents accordingly
fix
another fix
for node > 0.10 set an own agent with keepAlive

Set an Agent for requests.
keepAliveMsecs is also arangodbs default for connections.
Merge pull request #37 from baslr/master

set agent for node > 0.10