connection.py: SSLError raises NetworkError intead of SSLVerficationError.

Closes chfoo/wpull#91.
app.py: Removes implicit span-hosts enable.

Superseded by strong redirect logic.
url.py: Relaxes URL query percent-encode escapes.

Closes chfoo/wpull#67. Closes chfoo/wpull#90.
readme.rst: Uses SVG version of Travis badge.