Adding support for new functions in Lua 5.2 C API:
 - luaL_traceback
 - luaL_fileresult
 - luaL_checkversion (empty body, only to avoid compile errors)
'pcall' and 'xpcall' that allow calls to 'coroutine.yield', including support for a 'debug.traceback' that show the correct stack trace of the functions executed using these functions.
remove depedency of global variables, so the functions of module 'compat52' work even when the global functions are absent.
Updating README.md to describe the last changes.
Merge pull request #9 from renatomaia/master

'pcall' and 'xpcall' that support 'coroutine.yield'