XD-127 Add TCP Source

- Add 'binary' and 'charset' options to `file` sink
- Add TCP Source with selectable decoder
- Add documentation: https://github.com/SpringSource/spring-xd/wiki/Sources#wiki-tcp
Merge pull request #24 from garyrussell/XD-127

* garyrussell-XD-127:
  XD-127 Add TCP Source