Support for unicode

[tiojoca]

Some unicode characters and strings are not correctly read or written. This can be fixed by surrounding the decode with decodeURIComponent and escape when reading and doing the reverse on encoding when writing, as described in https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/btoa#Unicode_Strings

[johanneslumpe]

Feel free to submit a pr :)

[cjdell]

Unicode (UTF8) is now supported as the default encoding type for strings.