frame.hpp, md5.hpp, sha1.hpp x64 portability

[sedapsfognik]

Hello, guys!

I'm using your library for a while and got a lot of warnings which I did not care about.
But some of them seems very important as I think:

1>frame.hpp(831): warning C4267: '=': conversion from 'size_t' to 'uint32_t', possible loss of data
1>md5.hpp(367): warning C4267: '+=': conversion from 'size_t' to 'websocketpp::md5::md5_word_t', possible loss of data
1>sha1.hpp(176): warning C4267: '=': conversion from 'size_t' to 'unsigned int', possible loss of data

It is because I'm compiling for x64 platform and size_t is actually 64 bit, not 32 bit. You are using uint32_converter while there is another one presented: uint64_converter. I did not get deeper inside, but code, which calculates sha1 and md5 might be very sensitive to sizes difference.

Thanks!