Check whether interpreter changed and type (#123)

[dpalmasan]

This is an attempt to fix #123

The steps are the following

• Check virtualenv interpreter
• Check current interpreter
• If there is a match, do nothing. If they are different, recreate
• Check if the type of interpreter remains the same.

Not sure, but for the last case I made an assertion.

[theacodes]

Thank you!

[cjolowicz]

This caused a regression, see #424.

[dpalmasan]

Thanks, gonna check