Improve the RuleError type to include error codes.

This commit changes the RuleError type to a struct which consists of an
error code and human-readable description.

From a usage perspective, existing code should not break since type
asserting an error to a RuleError still works in the same manner.  The
difference is the caller can now take that type asserted RuleError and
access the .ErrorCode field on it to programmatically identify the
specific rule that was violated.

ok @jrick
Add tests for new RuleError and ErrorCode types.
Make orphan ntfns to a return val on ProcessBlock.

This commit changes the way that orphan blocks are identified by adding a
new boolean return value on ProcessBlock and removing the notification for
NTOrphanBlock.

This allows the calling code to identify orphan blocks immediately instead
of having to setup a seperate callback handler and implementing some type
of state tracking.  This, in turn, allows cleaner code for handling them.

In addition, the tests have been updated for the new function signature
and also now check that each block is or is not an orphan as expected
which makes the tests more robust.

ok @jrick
Make golint happy.
Modify ProcessBlock to accept behavior flags.

This commit change the ProcessBlock function to accept a new type named
BehaviorFlags in place of the current fastAdd parameter.

This has been done to pave the way for adding more control over the checks
that are performed such as avoiding the proof of work checks which will be
in an upcoming commit.  A bitmask was chosen because it will allow the
ProcessBlock API to remain a little more stable since new flag additions
that change the behavior are only likely to be used by new code because
adding flags does not change the existing behavior.

ok @jrick
Add a new behavior flag to disable the pow check.

This commit adds a new behavior flag, BFNoPoWCheck which allows the caller
to indicate the check which ensures a block hashes to a value less than
required target should not be performed.
Remove TODO that is complete.
Convert script errors to RuleErrors.

This commit modifies the error return type for errors during script
validation to use the RuleError so they are consistent with the rest of
the errors.  This also helps the calling code differentiate blocks
rejected due to script parsing and validation errors as opposed to
internal issues such as inability to read from the disk.

To accomplish this, two new two new RuleErrors, ErrScriptMalformed and
ErrScriptValidation, have been added.

Also, the errors for script parsing issues and script validation errors
have been improved to include both transaction hashes and indexes involved
in the validation effort.  Previously script parsing issues had almost no
additional information as to which transaction input/outputs the failing
script came from.
Update test coverage report.
Add a new behavior flag to provide a dry run.

This commit adds a new behavior flag, BFDryRun which allows the caller
to indicate all checks should be performed against the block as normal
except it will not modify any state.  This is useful to test that a block
is valid without actually modifying the current chain or memory state.

This commit also adds a few additional checks which were elided before
since they are implicitly handled by btcwire.  However, with the ability
to propose blocks which didn't necessarily come through the btcwire path,
these checks need to be enforced in the chain code as well.

As a part of adding the checks, three new error codes named
ErrBlockTooBig, ErrTooManyTransactions, and ErrTxTooBig have been
introduced.

Closes #5.
IMPORT PATH MODIFICATION CHANGESET
----------------------------------

Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.