Move BigNumber to internal

...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).
add deprecated bitcore.bignum and bitcore.base58

(the non-deprecated versions are bitcore.Bignum and bitcore.Base58)
fix firefox test
Merge pull request #301 from matiu/bug/firefox

fix firefox test
docco documentation
Merge pull request #303 from matiu/feature/docco-docs1

docco documentation
Merge pull request #298 from ryanxcharles/feature/bignum

Move bignumber.js to internal
add Address.forKey convenience method and example vanity address generator
name change forKey -> fromKey