Add signup endpoint about piccolo_api

[sinisaos]

@dantownsend I was thinking the same thing yesterday and it would be useful. Can I try implement this? My only question is how deep should we go with email and password validation without an external library. Is the html email field enough and is the 6-character password long enough? Or should we go deeper with regular expression?

[dantownsend]

@sinisaos Thanks, it would great if you could take a look. Just a simple regex should be OK for email validation. Even if the first version is very simple, it's still useful. I think we can add hooks at some point, so users can add additional logic for password and email validation.

[sinisaos]

@dantownsend I will make PR during the day that you can see code changes. We will drop a little bit in coverage because multiple lines like this one used to render errors in the HTML template. I think it's not a big deal.

[dantownsend]

Implemented in #141