Make ISA_BINOP and ISA_UNOP private

These are really implementation-specific details; not sure we want to
export them.
Use __all__ in bitcoin.core.scripteval

Previously was exporting the entire bitcoin.core.script namespace as
well.
Rename nMaxNumSize to MAX_NUM_SIZE

This is a constant; not meant to be changed.
Move non-consensus-critical constants out of bitcoin.core

Also deleted MIN_PROTO_VERSION and BIP0031_VERSION as the
python-bitcoinlib codebase doesn't even use them.
Use explicit import and __all__ in bitcoin.net

Previously was exporting bitcoin.core.serialize namespace.
Use __all__ in bitcoin.messages

First step in getting rid of the from foo import *'s
Pass Proxy() timeout parameter properly

Previously setting the timeout would never work as the super() call used
HTTP_TIMEOUT instead.
Prefix (HTTP_TIMEOUT|USER_AGENT) to DEFAULT_foo

Constants; changing them does nothing.
Prefix bord/bchr with _

Keeps them from being exported in module namespaces.
Prefix py2_(x|lx|b2x|b2lx) with _

Internal stuff
Stop exporting ssl and NID_secp256k1 in bitcoin.core.key

Internal details of the OpenSSL implementation used.
Stop exporting bitcoin.bloom.ROTL32

Implementation detail
Rename b58_digits -> B58_DIGITS

Is a constant
Add __all__ to all modules

Be explicit about what we're exporting everywhere.
Make bitcoin.core.bignum internal use

May be replaced, as was done in Bitcoin Core