Merge has line

Here is proposed implementation of `ensure_has_line` that is coircumvents shell escapes problem, but has own limitations (that is: it disallows """ in line). 

It relies on `execute_python_script` from latter commit, but I posted it as separate pull request. 

It is tested in the sense that I checked that it works, but I just fixed provy tests that failed because of `register_loader` in `core.Role`, and wrote no new tests. 

It works like that: 
1. Creates a python script that checks for line and append it if necessary
2. Uploads the script to remote server and executes it. 

What do you think about it?