Http Server: Write a text to a file

This adds the ability to write a text to a file.
The new op code WRITE will write the content to a file, if the overwrite
flag is set to true, the file will be truncated, if not, the text will
be append.

Calling PUT /file/f.txt?op=WRITE&overwrite=true&content=hello
will write hello to /f.txt overwrite its content if exist.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>