eval is unsafe, and the way it was being used in Script and Script interpreter
was not a good enough reason. This commit removes both uses of eval, then
replaces all uses of OP_XXX with Opcode.map.OP_XXX since there's no reason for
those constants to be global.
...no longer relies on Manuel's repo hostig a version of "bignum" that actually
contained bignumber.js. This moves bignumber.js internally and removes bignum
from the browser build process. Also adds a bitcore.Bignum that links to the
right thing. In node, browser.Bignum is require('bignum'). And in the browser,
bitcore.Bignum is is now Bignumber.js (same as before ... except bignumber.js
is now inside bitcore).
