Remove extra return statement found by go vet.

The return statement was unreachable and thus this change has no
effect on functionality.
Correct error message in handleGetWorkSubmission.

Found by go vet.
Initial pass at updating to btcnet.

This change modifies the params struct to embed a *btcnet.Params,
removing the old parameter fields that are handled by the btcnet
package.

Hardcoded network checks have also been removed in favor of modifying
behavior based on the current active net's parameters.

Not all library packages, notable btcutil and btcchain, have been
updated to use btcnet yet, but with this change, each package can be
updated one at a time since the active net's btcnet.Params are
available at each callsite.

ok @davecgh
Raise maxStandardSigScriptSize to 1650.

This commit raises the maximum allowed size for a standard signature
script to cover a 15-of-15 multi-signature pay-to-script-hash with
compressed pubkeys.

This mirrors a recent change to remain compatible with Bitcoin Core.

ok @owinga who also helped verify and correct the script math.

Closes #128.
Update pre-release and README.me to beta.

This is part of the changes for #130.
Prepare for release 0.8.0.