Fix infinite loop when executing some SQL statements with limit part

Issue #31 was about some problems in the SqliteTableModel which caused
an endless loop in the Execute SQL tab when executing statements with a
LIMIT part that didn't end with a semicolon. This was fixed in 0362d615fd
but the regular expression in this commit only worked for statements
like 'SELECT * FROM t LIMIT 0,10' and didn't detect a statement like
'SELECT * FROM t LIMIT 10' where there is only one number after 'limit',
so the bug still ocurred for those commands. This is fixed by this
commit.