added function to change the behavior of character encoding auto-detection.
when inserting file, check whole file to see if it is UTF-8.
implemented RFC 2231 filename encoding on send.
support nonstandard encoding 'ks_c_5601-1987'.
enhanced the support of environment-dependent Japanese characters.
changed the license of LibSylph from GPL to LGPL.
made some core modules library (libsylph).