Add const data access to message_t

Add a const version of data() and make size() const so we can pass messages around by
const refeference when we only need read-only data access.
Added 'connected' method to socket_t class.
Add explicit close method to context_t
Merge pull request #6 from ricnewton/context_close

Add explicit close method to context_t
Merge pull request #5 from ronelliott/master

Added connected helper method.
Merge pull request #4 from ricnewton/const_data_access

Add const data access to message_t
Make single parameter constructors explicit
Don't use deprecated functions
Use deleted functions if available
Merge pull request #12 from arsenm/master

Don't use the deprecated versions of functions
Merge pull request #10 from ricnewton/explicit-constructors

Make single parameter constructors explicit
Interface to zmq_proxy()
Correctly define ZMQ_DELETED_FUNCTION when using older clang compilers.

If you are using a version of clang that doesn't support cxx_deleted_functions
we need to define ZMQ_DELETED_FUNCTION to be empty.
Update zmq.hpp

work in progress...
added monitor_t class
added monitor() method
Update zmq.hpp

corrected monitor_t::on_event_unknown prototype
corrected indentation
work in progress...
syntax & bug fixing
modified monitor_t interface
Merge pull request #13 from jdswinbank/master

Adding a wrapper for zmq_proxy()
Merge pull request #14 from robertmaynard/master

Define ZMQ_DELETED_FUNCTION on older clang versions.
added possibility to unbind.