language changes: *T -> *const T.

this commit also fixes various places where an immutable value is
abused as a mutable argument to the SQLite function, which can be
very unsafe. the `sqlite3::ffi` module has been also audited and
revised to use mutable raw pointers whenever appropriate.
Merge pull request #86 from lifthrasiir/const-ptr

Language changes: *T -> *const T.
Fix bind_params off-by-one error

Sqlite3's parameter bindings are numbered from '1', not '0'.  This
allows `Cursor::bind_params` to be used.
Merge pull request #87 from d3zd3z/bind

Fix bind_params off-by-one error