Add an option for percent-encoding of the padding character to Base64Codec

Add an option for percent-encoding of the padding character to Base64Codec

Adds an encodePaddingCharacter parameter to Base64 codec and encoder.
Decode '%3D' as '=' in Base64 decoder


BUG=
R=sgjesse@google.com

Committed: https://github.com/dart-lang/crypto/commit/d4558dea1639e5ad2a41d045265b8ece270c2d90

[Alexander Ivanov, 2015-06-02 14:12:49]

alivanov@google.com changed reviewers:
+ sgjesse@google.com

[Søren Gjesse, 2015-06-03 11:32:04]

lgtm

https://codereview.chromium.org/1169453002/diff/1/lib/src/base64.dart
File lib/src/base64.dart (right):

https://codereview.chromium.org/1169453002/diff/1/lib/src/base64.dart#newcode98
lib/src/base64.dart:98: encodePaddingCharacter: _encodePaddingCharacter);
Long line. I will probably format it like this:

Base64Encoder get encoder =>
    new Base64Encoder(urlSafe: _urlSafe,
                      addLineSeparator: _addLineSeparator,
                      encodePaddingCharacter: _encodePaddingCharacter);

[Alexander Ivanov, 2015-06-03 11:56:13]

Committed patchset #2 (id:20001) manually as
d4558dea1639e5ad2a41d045265b8ece270c2d90 (presubmit successful).