Add bindVars to allowed keys for cursor path

Added bindVars to ALLOWED_KEYS_FOR_PATH so the Query#prepare_request
method does not end up stripping out bound variables.
Adjust hound config
Added @sleepycat's example for bind variables to the execute doc
Merge pull request #109 from triAGENS/sleepycat-bindvars

Add bindVars to allowed keys for cursor path [Reviewed]