Don't escape the request body in HTTP listen input #1124
Add configuration flag for backward compatibility

There is a new configuration option for HttpListenInput called
unescape_body. When true, the request body is unescaped before being
injected into the payload. The default value is true so it changes
nothing compared to the previous behavior.
Merge branch 'dev' of https://github.com/simonpasquier/heka into simonpasquier-dev

Conflicts:
	plugins/http/http_listen_input_test.go
Merge branch 'simonpasquier-dev' into dev