Review/comment needed for the new public java.util.Base64 class

[Xueming Shen]

A standard/public API for Base64 encoding and decoding has been long
overdue. JDK8  has a JEP [1] for this particular request.

Here is the draft proposal to add a public Base64 utility class for JDK8.

http://cr.openjdk.java.net/~sherman/4235519/webrev

This class basically provides 4 variants of Base64 encoding scheme,

(1) the default RFC 4648, which uses standard mapping, no line breaks,
(2) the URL-safe RFE 4648, no line breaks, use "-" and "_" to replace 
"+" and
     "/" for the mapping
(3) the default MIME style, as in RFC 2045 (and its earlier versions), 
which uses
     "standard" base64 mapping, a 76 characters per line limit and uses 
crlf pair
      \r\n for line break.
(4) an extend-able MIME base64, with the char-per-line and the line 
separator(s)
      specified by developer.

The encoder/decoder now provides encoding and decoding for byte[], String,
ByteBuffer and a pair of "EncoderInputStream" and "DecoderOutputStrream",
which we believe/hope should satisfy most of the common use cases. 
Additional
method(s) can be added if strongly desired.

We tried couple slightly different styles of design for such this 
"simple" utility
class [2].  We kinda concluded that the version proposed probably provides
the best balance among readability, usability and extensibility.

Please help review and comment on the API and implementation.

Thanks!
-Sherman

[1] http://openjdk.java.net/jeps/135
[2] http://cr.openjdk.java.net/~sherman/base64/