OpenPop.NET 2.0.1 released

This is mostly a bugfix release. OpenPop.NET 2.0 was a major upgrade but unfortunately some bugs were found, some of which were severe enough to make a new release.

Changes:
Fixed a bug where _ was being converted to spaces if encoded using QuotedPrintable.
Fixed a bug where special RFC 2231 decoding would fail.
More robust parsing of dates.
More robust parsing of Content-Transfer-Encoding header.
Now sending QUIT when Pop3Client is disposed.
Other small fixes.
Also CAPA command and Cram-Md5 authentication is now supported.
Other small refactorings.