Add new funcs for creating payment scripts.

This commit adds two new functions named PayToScriptHashScript and
PayToAddrScript.  The first one creates and returns a public-key script
which pays to the provided script hash and conforms to BIP0016.

The second function takes the new btcutil.Address interface type and
returns an appropriate script to pay to the address type in the interface.
It currently works for btcutil.AddressPubKeyHash and
btcutil.AddressScriptHash.
Add tests for new payment script creation funcs.
Modify DisasmString to return partial disassembly.

Rather than returning an empty string from DisasmString if a script fails
to parse, return the disassembly up to the point of the failure along with
[error] appended.  The error is still returned in case the caller wants
more information about the script parse failure.
Group the StackErrX definitions.

This improves the godoc display and provides a cleaner separation of the
error definitions from other variables.
Change OP_CHECK_MULTISIG -> OP_CHECKMULTISIG.

This change was made to match the reference implementation.
Fix comment typo.
Make opcode disasm names match reference impl.

This commit modifies the names of opcdoes shown in the oneline script
disassembly to match the reference implementation.  In particular
OP_1NEGATE, and OP_0 through OP_16 are changed to the raw numbers
they represent when doing oneline disassembly.  When doing full
disassembly, the full opcode names are still shown.

ok @owainga.
Refactor and improve address extraction code.

This commit significantly changes the address extraction code.  The
original code was written before some of the other newer code was written
and as a result essentially duplicated some of the logic for handling
standard scripts which is used elsewhere in the package.

The following is a summary of what has changed:

- CalcPkScriptAddrHashes, ScriptToAddrHash, and ScriptToAddrHashes have
  been replaced by ExtractPkScriptAddresses
- The ScriptType type has been removed in favor of the existing
  ScriptClass type
- The new function returns a slice of btcutil.Addresses instead of raw
  hashes that the caller then needs to figure out what to do with to
  convert them to proper addressses
- The new function makes use of the existing ScriptClass instead of an
  nearly duplicate ScriptType
- The new function hooks into the existing infrastructure for parsing
  scripts and identifying scripts of standard forms
- The new function only works with pkscripts to match the behavior of the
  reference implementation - do note that the redeeming script from a p2sh
  script is still considered a pkscript
- The logic combines extraction for all script types instead of using a
  separate function for multi-signature transactions
- The new function ignores addresses which are invalid for some reason
  such as invalid public keys
Add 100% test coverage for ExtractPkScriptAddrs.
Add 2014 to copyright dates.
Remove warnings for errors that are returned.

ok @owainga.
Fix a number of bugs in signature checking

When given badly formatted  signature or pubkeys like in block
0000000000000001e4241fd0b3469a713f41c5682605451c05d3033288fb2244, transaction
fd9b541d23f6e9bddb34ede15c7684eeec36231118796b691ae525f95578acf1 we could fail
on strange scripts because we returned an error instead of failing the
opcode and putting a FALSE on the stack.

Fixes chainfork issue on the aforementioned block.
Gofmt.
gofmt