add proper DER signature support to Key

both creating DER signature from the r and s values, and parsing a DER
signature into the r, s, and other properties.
remove obsolete Curve class (G and n now accessible from Point)
expose signature internal functions

"sign" and "genk" ... and add some signature tests
add further tests for Key
require SecureRandom and Point ... woops
move bitcore-dev.js -> bundle.js

To make it easier to test the master branch of bitcore with other apps,
especially Copay.
Merge pull request #440 from ryanxcharles/feature/der-signatures

DER format signatures