Replace custom quote function with shlex.quote

[pbhandari]

The custom written shell.quote function was causing issues with it not
correctly escaping some characters. As Python 3.3+ comes with a quote
function in the shlex module, we use that in place of the custom
quote function.

Fixes issue #97.

[pbhandari]

@WtfJoke: if you're still skeptical about using shlex.quote wholesale, I could just modify the shell.quote function to escape out the $

[WtfJoke]

Sorry about my late response. I'll have a look at it during the next weeks/days.
Quite busy lately.

But I think we can merge that without any problems.
Thanks for your contribution Prajjwal! :)

[WtfJoke]

Looks good, thank you :)

[dentych]

I would like to say, that this shlex.quote thing completely messed up the script for me. I had to remove all shlex again.

It was executing commands like: scm login -r -u '"'"''"'"' -P '"'"''"'"'

I manually replaced all shlex, and put in " everywhere. That worked.

[WtfJoke]

Ok, thank you for the info. I will revert this.

[WtfJoke]

I would like to say, that this shlex.quote thing completely messed up the script for me. I had to remove all shlex again.

I reset the code to the point without merging this pull request