do not install rust-nightly via apt-get
language changes: std::str::raw::from_c_str is deprecated; fixed some testing issues.
revamped `Cursor::get_{text,blob}` functions. closes #92.

they used to return a direct `String` or `Vec<u8>`, which may fail
when it should return `NULL`. other functions do not have this issue
since the automatic conversion is fully defined over INTEGER and
FLOAT internal types.

this commit changes them to return an optional `&str` or `&[u8]`.
this correctly handles `NULL`, and moreover, reduces the redundant
allocations when the buffer doesn't need to be kept. this also
makes `Cursor::get_bytes` fully redundant, so it has been removed.
Merge pull request #94 from lifthrasiir/get-text-madness

Revamp `Cursor::get_{text,blob}` functions
elaborate prepared_stmt_bind_static_text to illustrate with_c_str() bug #95

--HG--
extra : source : adc9264cba0d828aaf7815c538174ea7b74f1c49
fix string lifetime in StaticText case of bind_param.
fixes: #95

--HG--
extra : source : 89e2a09be1e08cc8ddb3c5688bae670ce0d2ab53
Merge pull request #97 from dckc/bind-static-text-95

fix StaticText lifetime bug in bind_param (#95)
fixes various mutability problems.

- fixed a mistake that `Database` was accidentally `Send`able.
- made `Cursor::{reset, clear_bindings, step, step_row,
  get_blob, get_int, get_i64, get_f64, get_text, bind_param,
  bind_params}` receive `&mut self`.
- made `Database::set_busy_timeout` receive `&mut self`.
  `Database::{prepare, exec}` has been left as is, since it is
  perfectly safe to have multiple cursors. the associated
  thread safety issue is separately handled.

closes #98, #99, #100, #101 and #103.
Merge pull request #104 from lifthrasiir/mutability-fix

Fix various mutability problems.
remove the Travis badge of my fork
addressed the excessive kinds on `Database`. fixes #103.

- `Database` is now `Send`able, but it remains non-`Copy`able
  and non-`Sync`able. thus this change only permits `Database`
  to be *exclusively* shared across tasks.
- while it is not strictly necessary, @apoelstra pointed out that
  leaving `Database::exec` to only require an immutable reference
  may result in a surprising behavior. as a conservative measure,
  this commit also changes it to require a mutable reference.