Do UTF-8 encoding natively.

For strings consisting exclusively of ASCII characters, this is slightly
faster on the desktop JVM:

    length     benchmark  ns linear runtime
        64 NativeEncoder 103 ===
        64      GetBytes 113 ===
      1024 NativeEncoder 894 ============================
      1024      GetBytes 933 ==============================
    vm: java

It's substantially faster on dalvikvm:

    length     benchmark    us linear runtime
        64      GetBytes  6.46 ===========
        64 NativeEncoder  1.47 ==
      1024      GetBytes 16.20 ==============================
      1024 NativeEncoder 13.58 =========================
Merge pull request #87 from square/jwilson_1130_utf8_transcoder

Do UTF-8 encoding natively.