Factor out key generation

There's no need to specify the curve all over the place.  Just make
special case functions which use the Bitcoin curve.
Elliptic curve blind signatures

If you replace secp256k1 with P224 (or another curve supplied by the
native Go library), then examples/blind.go works perfectly.  Of
course, since Go's elliptic curve library uses different curves than
secp256k1, using those curve params doesn't work at all.  I'll address
that shortly.
Run 'go fmt'