Remove deprecated address functions.

This commit removes the deprecated address functions which have been
replaced by the Address interface and concrete implementations.
Consolidate remaining addrconvs.go to address.go.

Since all of the deprecated address conversion functions have been
removed, consolidate the remaining private key funcs and tests into
address.go and address_test.go, repectively.
Rename DecodeAddr to DecodeAddress.

Now that the deprecated DecodeAddress has been removed, rename DecodeAddr
to DecodeAddress and remove the comment which warned this change was
coming.
Modify DecodeAddress to accept serialized pubkeys.

This commit modifies DecodeAddress to accept and decode pay-to-pubkey
addresses (raw serialized public keys).  Since the resulting Address
needs to have a network associated with it, and a raw serialized public
key does not encode the network with it, a new parameter has been added
which requires the caller to specify a default network to use when
decoding addresses.

In the case the address has a network encoded with it such as for
pay-to-pubkey-hash and pay-to-script-hash addresses, the network will be
decoded from the address and the resulting Address instance will have that
network associated with it.  When the address does NOT have a network
encoded with it, such as a pay-to-pubkey address, the provided default
network will be associated with the returned Address instance.

Also, the tests have been updated to test the new functionality.

ok @owainga and @jrick.