add stdin option, write to stdout in python 2/3, handle remote urls

adds ability to convert files with remote input url, 
checks python version before writing to stdout and uses buffered write for python 3 and direct write for earlier versions,
adds stdin option using private:stream
still works with regular file input/output as well


##### -r or --remote option to use remote url.
```
root@app0:~# unoconv -r --stdout -f html http://fqdn.com/sample.docx
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
...
```
##### --stdin option to read input from stdin. 
```
root@app0:~# cat sample.docx | unoconv --stdin --stdout -f html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
...
```