Added ability to enumerate ports on linux and windows.

Updated serial_example.cc to show example of port enumeration.
return a vector<array<string, 3> > from list_ports

instead of a vector<vector<string> >
Refactored list_ports to return a vector of serial::PortDescription.
Changed the serial::PortDescription struct.

- Renamed to PortInfo.

- "friendly_name" field is now "description".
Added osx implementation to list_ports.
Updated documentation.
Fixed a syntax error.

In list_ports_win.cc.
Merge pull request #59 from LilleySavij/list_ports

List ports