Fix issue 112: perl encoding pragma deprecation.

Replaces the 'encoding' pragma with the 'utf8' pragma. The former is
deprecated under perl 5.18 and higher and may be removed in a future
version.

  - http://perldoc.perl.org/encoding.html
  - http://perldoc.perl.org/utf8.html
Apply patch (issue 110): Respect $XDG_CONFIG_HOME

  - Submitted by https://code.google.com/u/111076641406023763561/
  - Submitted at https://code.google.com/p/abcde/issues/detail?id=110
Remove explicit reference to XDG_CONFIG_HOME path.

Although if a user sets XDG_CONFIG_HOME it's likely they'll interpret
the documentation correctly, it's perhaps better not to explicitly
refer to the ~/.config directory; "hardcoding" the path in the
documentation seems counter to the flexibility of the XDG Base
Directory Specification. Here, the files are referred to as "system"
and "user" configuration files with an extended explanation of where
these are in the man page.
Respect XDG_CACHE_HOME.

  - If on OSX, keep existing behaviour of outputting to current
    directory. I'd be guessing if I tried to implement proper
    directories (see http://stackoverflow.com/q/3373948).
  - Otherwise, the default (for an unset/empty XDG_CACHE_HOME) is:

        ~/.cache/abcde/abcde.blah-blah-discid
Respect XDG_CONFIG_HOME (Patch in issue 110)
Respect XDG_CACHE_HOME (issue 90)