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.
This is more flexible; maybe the scriptSig being passed is some new form of signed message, for example. But it would make sense to accept NULL to do the extraction from txTo...