Updated to use an unordered_map.

  This commit adds a hash-map for storing key=value settings,
 and updates the get_maildir_prefix/set_maildir_prefix functions
 to use it.

  This is part of the code required for issue #20.

  Rather than updating all the get/set_foo() functions we should
 remove them all and switch to using get_variable() and set_variable().

  Ideally we'd also expose the variables to lua.