Initial esp_service for tcp sockets

Collected commit of initial attempt to handle asynch tcp sockets.
Introduces the following concepts:

  - esp_service: a behaviour for start/stop control of a service
  - esp_tcp_service: async tcp socket listening as an esp_service
  - test suite for both

The modules esp_http, esp_tcp_async are earlier failed attempts
which will be removed in the process of making an esp_epmd_service.

The esp_tcp_service hands incoming packets to a Client_Module that
must be a behaviour implementing recv/1, recv_error/1 and recv/closed/1.

The interface will change to include the socket, a behaviour module
will be added and other enhancements before esp_tcp_service can be
considered experimentally implemented.
Merge pull request #6 from jaynel/features

Initial esp_service for tcp sockets