Rewrite script/bootstrap in Python

This is the first step toward making our scripts work on Windows.
Rewrite script/update in Python

Another step along the road to Windows compatibility.
Rewrite script/build in Python
Rewrite script/create-dist in Python
Rewrite script/upload in Python
Rewrite script/cibuild in Python
Rewrite script/download in Python

Note that we no longer use curl and unzip to download/extract files,
opting to use built-in Python functionality instead.
Merge pull request #5 from aroben/python

Rewrite scripts in Python