Updated to test for strings before using them.

  This updates #182.
  Test all arguments for string-ness prior to fetching.

  This updates #182.
  Updated bindings_file.cc to test for string-ness.

  Test all string-arguments to make sure they're strings prior
 to using them.  Updates #182.
 Updated bindings_folders.cc to test for string-ness.

  Test all string-arguments to make sure they're strings prior
 to using them.  Updates #182.
  Updated bindings_index.cc to test for string-ness.

  Test all string-arguments to make sure they're strings prior
 to using them.  Updates #182.
  Updated bindings_maildir.cc to test for string-ness.

  This updates #182.
  Update binding_message.cc to test for string-ness.

  Test for strings prior to using them.  Updates #182.
  Update bindings_mime.cc to test for string-ness.

  This updates #182.
  Updated to test for string-ness.

  Also test that `show_file_contents` has a valid file, ie. one that
 exists.

  This updates #182.
  Test that arguments are the correct type.

  This closes #182.
  Updated to compile.
  Added `on_error` callback.

  When a syntax error, or other lua error, is encountered we
 invoke the function `on_error` if it is defined.

  This closes #183.

  Documentation:

    http://lumail.org/lua/on_error.html
Support DESTDIR, for easier (Mac)porting
Merge pull request #185 from xipmix/master

Support DESTDIR, for easier (Mac)porting
  Updated learning-lua link to "Learn Lua in 15 minutes"

    http://tylerneylon.com/a/learn-lua/
  We could use luaL_checkstring for simpler code.
  Only close the domain-socket if missing.
  Dump attachment names too.
Use PREFIX portably.
 - bin/ dirs should come at PREFIX/bin, etc
 - set it to /usr if not already set
Merge pull request #188 from xipmix/master

Use PREFIX portably.