Add css_text option to allow css strings to be provided

[jonbretman]

This adds a new option to Premailer - css_text - which allows you to pass some css in directly that should be applied to the given html.

This is very useful if you are sending thousands of emails, each one with personalised content, but that all use the same CSS file. The css file(s) can then be read once and cached and passed directly to Premailer each time, avoiding any additional disc/network IO.

Before

for recipient in recipients:
    html = render_html(recipient)
    # will read from disc every time
    html = Premailer(html, external_style='style.css').transform()
    send_email(html)

After

# read from disc once
css = read_file('style.css')
for recipient in recipients:
    html = render_html(recipient)
    # no disc IO needed
    html = Premailer(html, css_text=css).transform()
    send_email(html)

Additionally fixed some pep8/flake8 warnings.

[coveralls]

Coverage remained the same when pulling 5d87311 on jonbretman:support-css-text into c7b40c0 on peterbe:master.

[peterbe]

This is looking promising. But I don't want to merge it until we fix that Travis runs the tests across all the known versions of Python.

See #89

[coveralls]

Coverage remained the same when pulling 5d87311 on jonbretman:support-css-text into c7b40c0 on peterbe:master.

[peterbe]

Yay! It works in all versions of Python.