Base64 加密算法（Author 蒋柏良 Jerry Jiang)-*

* DEscr iptION

*　　Base64 processes input in 24bit chunks by converting each chunk into 4

*　　bytes of output. It does so by splitting input into four 6bit groups and

*　　using these as indexes in the following substitution table-

*　　　const char base64_map[] =

*　　　　　　　　"ABCDEFGHIJKLMNOPQRSTUVWXYZ"

*　　　 　　　　"abcdefghijklmnopqrstuvwxyz"

*　　　　　　　"0123456789+/"

*　　If an input is not a multiple of 3 bytes, it s padded with zeros. In

*　　this case the output bytes that consist entirely of the pad data are

*　　replaced with　= .

*

* Example

* 　　An input of 0x00 0x45 0xF2 is equivalent to 00000000 01000101 11110010

* 　　bit sequence, which s then split into 000000 000100 010111 110010

* 　　and these are substituted to produce the following base64 encoding

*　　　 A　 E　 X　 y

*

* RETURN VALUE

* 　　b64_length() should return output length for the input of　len　bytes.

* 　　b64_encode() should encode　len　bytes from　src　into　dst .　dst　is

* 　　guaranteed t