add contract.gasLeft()

[kroggen]

This PR adds the contract.gasLeft() function to retrieve the remaining gas in the contract execution

Closes #239

There is a problem though:

The lua_pushinteger() function accepts a value of type lua_Integer, that is signed int64, and the lua_gasget() function returns an unsigned int64

So if the value of gas is too big, it may appear for the Lua code as a negative value (to be tested)