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