Read aliases file as bytes and convert to string

Discovered that my Mutt aliases file uses the latin1 character encoding.
That caused a "stream did not contain valid UTF-8" error when trying to
read the file in the `Alias#find_in_file` function.

This error was ostensibly triggered by a `str::from_utf8` call in the
standard library
(https://github.com/rust-lang/rust/blob/2174bd97c1458d89a87eb2b614135d7ad68d6f18/src/libstd/io/mod.rs#L315-L338).

I ended up finding this Stack Overflow answer with an easy solution:
http://stackoverflow.com/questions/28169745/what-are-the-options-to-convert-iso-8859-1-latin-1-to-a-string-utf-8/28175593#28175593

      fn latin1_to_string(s: &[u8]) -> String {
          s.iter().map(|c| c as char).collect()
      }

Since latin1 is a subset of Unicode, we can just read the bytes from the
file and typecast them to Rust chars (which are UTF-8). That gives us
the opportunity to easily get the text into an encoding that we can
actually work with in Rust.

At first I got frustrated because the suggestion didn't compile for me.
It was suggested in January 2015, before Rust 1.0, so perhaps that
factors into the error I was getting. Here it is:

    src/alias.rs:59:41: 59:45 error: mismatched types:
     expected `&[u8]`,
        found `core::result::Result<collections::string::String, std::io::error::Error>`
    (expected &-ptr,
        found enum `core::result::Result`) [E0308]
    src/alias.rs:59             let line = latin1_to_string(line);
                                                            ^~~~
    src/alias.rs:59:41: 59:45 help: run `rustc --explain E0308` to see a detailed explanation
    src/alias.rs:99:22: 99:31 error: only `u8` can be cast as `char`, not `&u8`
    src/alias.rs:99     s.iter().map(|c| c as char).collect()
                                         ^~~~~~~~~
    error: aborting due to 2 previous errors

A recommendation from 'niconii' Mozilla#rust-beginners was to use the
Encoding library in order to do the conversion
(https://github.com/lifthrasiir/rust-encoding). That certainly seems
more robust and would be a good idea to try if this change doesn't work
out in the long term. But the Stack Overflow answer just seemed so short
and sweet that I really didn't like the idea of adding a dependency if I
could get what I wanted with 3 lines of code.

Finally took another look and reworked the suggested code to take a
vector (which is what `BufReader#split` gives us) and clone the u8
characters to clear the compiler error of not being able to cast an &u8.

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