Logic changes for hooks / hdrs / buffers

Since request/connection structures are now hidden, the following accessor
functions were added:

- evhtp_request_buffer_out()
- evhtp_request_buffer_in()
- evhtp_request_get_headers_out()
- evhtp_request_get_headers_in()

Since hooks can no longer be accessed directly, the following functions were
added to set specific hooks on requests / connections / callbacks:

- evhtp_request_set_hook()
- evhtp_connection_set_hook()
- evhtp_callback_set_hook()
Added examples/CMakeLists.txt