add proxy class for mysqli

By moving the error check here, we can save an immense amount of code.
add $msg parameter to proxy_mysqli::query()
fix items/add for new handling in proxy_mysqli
minor DB error message fix in proxy_mysqli
Remove error checks after MySQL query

The `proxy_mysqli` class now handles all errors where mysqli::query()
returns FALSE. This eliminates the need to check for this case in the
calling code.
Further shorten code concerning MySQL operations

Move the check for at least n (usually 1) results from
a SELECT query to the Assert::dbMinRows() method. This
shortens and simplifies a lot of code.
Also use Assert class for non-SELECT operations

Enable Assert::dbMinRows() to accept a mysqli instance after
an operation such as INSERT, UPDATE or DELETE. In this case,
the affected_rows property is checked, not num_rows as after
SELECT operations when a mysqli_result instance is passed.
Merge pull request #33 from maul-esel/proxy_mysqli

code shortening for mysqli handling