lib: Add a password API

Issue #50.
db.php: Add a pip_db_num_rows() function

Issue #50.
register.php: Sanitise POST strings

Issue #50.
setup.php: Create an admin account type

Issue #50.
config.php: Fix table creation order

Issue #50.
accounts.php: Implement new account adding

Issue #50.
db.php: Add a pip_db_fetch_row() function

Issue #50.
password.php: Minor style fix

Issue #50.
password.php: Add a pip_password_match() function

Issue #50.
string.php: Fix function name

Issue #50.
db.php: Throw an exception if pip_db_fetch_row() has no results

Issue #50.
accounts.php: Implement login functionality

Issue #50.