Initial version of TCPSocket_HelloWorld sample.

As exported from http://mbed.org/users/mbed_official/code/TCPSocket_HelloWorld/
after upgrading to latest EthernetInterface and mbed-rtos sources.

NOTE: This version doesn't work with gcc4mbed but will be modified in upcoming
      commit to do so.
Update to latest Socket library as well.
Updated TCPSocket_HelloWorld sample to work with gcc4mbed.

The biggest change was to software_init_hook() to properly fill in the
stack size and initial stack pointer when creating the intial main thread.
Add TCPSocket_HelloWorld to the build list.