libbitcoinscript

This does the minimal effort to generate a libbitcoinscript.so which can VerifyScript, though nothing else is exposed.

Build with ./configure --enable-shared=yes and you'll find a libbitcoinscript.so installed (or in src/.libs).

It weighs quite a bit, but it works, so I think its worth merging and then trimming down (and adding features) as we go (Ive already got one or two others ready, but first things first).

Please do not ACK this unless you've verified there are no changes before/after the copy/paste.
We already have several functions implemented in core.h, including core.cpp doesnt just pull in core.cpp, but quite a few others quite quickly. The library weighs a lot now, but including core.cpp would end up with half of bitcoind...
re: modifying the header file and breaking build? That's what we have pull-tester (or Travis) for, they tell you if you break something dumb.