Major rewrite of the contents portion of Horde_Mime_Part.

Major rewrite of the contents portion of Horde_Mime_Part.

Always store data inside part in binary format. Reduces overhead (no
need to store in larger base64 format) and allows us to do the
decompression at the same time we have to write the data to the new
internal stream object anyway.

Reworked parameters for setContents(), appendContents(), getContents(),
setTransferEncoding(), addMimeHeaders(), toString().

Removed getContentsAsStream(), getCanonicalContents() -> these can be
accessed via options to getContents().

Removed transferDecode(), transferEncode(), transferEncodeContents(),
transferDecodeContents(), strict7bit(), getTransferEncoding(), and
getCurrentEncoding() -> this is all handled internally on a call to
toString().

Removed toCanonicalString() -> this can be accessed via options to
toString().

Also:
Fix rtrim() call (don't trim non-EOL whitespace chars).
phpdoc