src: fix unaligned access in ucs2 string encoder

Seen with g++ 4.9.2 on x86_64 Linux: a SIGSEGV is generated when the
input to v8::String::NewFromTwoByte() is not suitably aligned.

g++ 4.9.2 emits SSE instructions for copy loops.  That requires aligned
input but that was something StringBytes::Encode() did not enforce until
now.  Make a properly aligned copy before handing off the input to V8.

We could, as an optimization, check that the pointer is aligned on a
two-byte boundary but that is technically still UB; pointers-to-char
are allowed to alias other pointers but the reverse is not true:
a pointer-to-uint16_t that aliases a pointer-to-char is in violation
of the pointer aliasing rules.

See https://code.google.com/p/v8/issues/detail?id=3694

R=@trevnorris?  Please disregard the first commit, that's #125.