Depend on struct instead of lpack

The struct module is actually available either for Lua 5.1 and 5.2.
lpack was the only dependency preventing lua-websocket to be compiled
for Lua 5.2.
Missing compatibility for Lua 5.2 bit operations

In the standard library provided by Lua 5.2, bit.rol and bit.rol are
named bit32.lrotate and bit32.rrotate.
Merge branch 'pr1'
Merge branch 'pr2'
Leverage luasocket for base64 encoding

luasocket is already a dependency of lua-websockets so its base64
encoding routine can be used instead of the hand-crafted one:

http://w3.impa.br/~diego/software/luasocket/mime.html#b64
Merge branch 'pr3'