ENHANCED: Wrap pengine events into pengine_event(Id, Event), so we can
wait for events of a specific pengine.  Needed for pengine_rpc/2.
MODIFIED: Introduce per-application settings.
DELETED: setting max_session_pengines
ADDED: Setting slave_limit
FIXED: Pass application option properly to remote pengine create
MODIFIED: Reply normal pengine error if a remote pengine does not exist
FIXED: handle errors in creating remote pengines.
Disable counting childs for pengines created from HTTP server threads
ADDED: Manage pengine resources using thread pools
MODIFIED: HTTP messages are no longer wrapped in request/1 terms.  This
is the visible consequence of separating messages to pengines from their
replies.
TEST: Added regression testing for the web client
Make pull-request safe.
Added test about creating childs and interacting with them
Test: make library(pengines) available to the application
Test for phantomjs