Unicode support

[smarek]

With #5 Unicode module for handling utf8 <-> utf16 <-> unicode has been added, with CP437 and SJIS (JIS X 0208 Appendix 1)

• Separate unicode functions from included encoding tables
• Merge unicode.lua and string_decode.lua
• Define codetables, that should be included in runtime
• Define how to extend the runtime with custom codetable (functions in unicode module will help greatly, dec/enc pattern is already present there)