Updated BLPAPI to 3.7.9.1 (Linux/Windows), 3.8.1.1 (OS X).

Updated documentation to refer to the new version numbers and URLs
for the website and documentation.
Merge pull request #27 from apaprocki/blpapi-3.7.9.1

Updated BLPAPI to 3.7.9.1 (Linux/Windows), 3.8.1.1 (OS X).
Fix leaking blpapi::Session.

The underlying `blpapi::Session` can not be deleted from within a
message dispatch, so instead indicate that `destroy()` was called
and delete the `Session` outside of the processing loop, taking
care to destruct the `MessageIterator` and drain the queue of any
`Event` objects first, as they require the `Session` to exist.