Removed String converters

[kkoopa]

NanUtf8String and NanUcs2String just duplicate v8's equivalent classes. NanAsciiString is not really an ASCII string, true ASCII is a subset of UTF-8, so that can be used instead.

[kkoopa]

Merged in efced26.

[springmeyer]

@kkoopa - are you sure this is uneeded? I'm trying to port https://github.com/mapnik/node-mapnik/blob/a40d9ce8c6c44f201c5319d84c34d77f79072a9a/src/mapnik_palette.cpp#L45-L46 to Nan 2.0.5 and after switching to Nan::Utf8String I'm seeing test failures. It may be on my end, but I thought I'd let you know just in case you have ideas. I wonder if v8's string->WriteAscii behaves differently than string->WriteUtf8.

[kkoopa]

They behave very differently. The old so called Ascii methods dealt in raw bytes with no actual encoding. This was rectified. The difference here is that you are not passing a valid ASCII string. Since ASCII is a subset of UTF-8, a valid ASCII string is valid UTF-8. Use a Buffer instead.

[springmeyer]

@kkoopa - Thank you! That fixed it :)