Fix broken docs and examples when some rusqlite features are used

[cljoly]

WIP PR to fix the problem raised in #15 (comment):

Hi there, first of all thanks for your work!
I just stumbled across this line on the top level documentation of the crate:

conn.pragma_update(None, "journal_mode", &"WAL").unwrap();

This panics for me with ExecuteReturnedResults. According to this issue in rusqlite we have to use pragma_update_and_check instead.

So probably the easiest way to fix the documentation would be to change that line to:
conn.pragma_update_and_check(None, "journal_mode", &"WAL", |_| Ok(())).unwrap();

[coveralls]

coverage: 95.796%. remained the same
when pulling e4d489c on fix-docs
into abc7991 on master.