Added control connection

Also change the LoadBalancingPolicy interface to use an iterator
design so that we can eventually optimize for large numbers of hosts.
Added copy on write mechanism

This will prevent us from having to fully copy the hosts list for
every query plan. I've also updated the RefCounted object to use
release/acquire memory ordering semantics which will likely perform
much better than the  sequential consistent memory ordering we were
using before. Here's the references I used:

http://www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html#boost_atomic.usage_examples.example_reference_counters
http://www.boost.org/doc/libs/1_55_0/doc/html/atomic/interface.html#atomic.interface.interface_memory_order

COW:
http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Copy-on-write
Added mechanism to run queries on the control conn

Fixed issue where the connection wouldn't have been
closed as a result of a timeout or error.
Merge pull request #56 from datastax/CPP-85

CPP-85 Add control connection
Added way to register events
Add from_inet for initializing Address
Added event response type
Merge remote-tracking branch 'origin/CPP-145' into CPP-146
Change to use Address in IOWorker backend

Changing from Host to Address in backend, where extended Host attibutes
should not be needed.
Add some missing license headers
Further conversion of Host-->Address in low-level code

Connection updated
Avoid pool ready callbacks on reconnect.

Prevents the Session::pending_pool_count_ invariant from going negative, when
it should really only be used to count down initial connection attempts.
Control connection cluster node discovery.
don't leak ResultIterator in control connection
Cleanup leaks in a few examples.
Improved error handling

Updates from code review:
- Handle ErrorResponse, unexpected opcode
- Note on queries about dependent indexing
- Minor formatting
Added ability to register for events
Merge pull request #57 from mfiguiere/beta4
Merge pull request #59 from datastax/CPP-145
Short-circuit control startup on auth errors