Force UTF8 for all text reading / writing.

Replaced all usages of readFile, writeFile, hGetLine, and hPutStrLn from
Prelude with versions from
[System.IO.UTF8](http://hackage.haskell.org/package/utf8-string-0.3.8/docs/System-IO-UTF8.html)

Fixes #94

Tested with `export LANG=C`. Was able to load a unicode-containing .idr
file, compile and run it, use interactive vim stuff like
case-splitting and proof search, and could also :addproof from the repl.

Technically, only the change to readFile in Idris/Chaser.hs is necessary
to fix #94.  The rest are mainly for consistency.  I am a little leary
of changing the output encoding to unicode when the system encoding
might not be that on the things that don't deal with idris code, e.g.
the .c, .java, .pom, etc output files, that are then run through gcc,
javac, whatever.

Also, this might address the issue fixed by #1334, and make #1334
redunant?