Mark getchaintips RPC command as unimplemented.
Fix the mempool rate limiter.

Before, btcd was rate limiting all transactions that had a minimum
fee of zero.  Now, btcd only rate limits transactions that contain
a fee less than the calculated fee based on size.

Closes #163
Improve readability of recent rate limiter changes.

This commit builds on the recent limiter updates.  The recent changes
introduced a new function named calcTxFee which could easily be confused
with calculating the fees of the transaction rather than its intended
purpose of calculating the minimum transaciton relay fee.

Rather than taking that approach, this commit instead renames the existing
function to calcMinRequiredTxRelayFee and uses a consistent variable name
for the serialized size of a transaction.  It then moves the check for
whether or not the check should be applied based on the serialized size of
the transcation and block priority to the call site.

This approach also has the benefit of avoiding two calls to the
calculation function since it's a local at the call site.

ok @jrick, @dajohi
Always show help on help flag.

This commit resolves a minor issue where an error in the config file would
prevent the help from being shown until the config file error was
resolved.

This results in the following behavior:

- With a malformed header:
  $ ./btcd
  Error parsing config file: ~/btcd/btcd.conf:14: malformed section header
  Use btcd -h to show usage
- With an invalid option:
  $ ./btcd
  Error parsing config file: unknown option: bogus
  Use btcd -h to show usage
- Invoking help with an error in the config file:
  $ ./btcd -h
  Usage:
  ...
- Invoking with an invalid command line option:
  $ ./btcd --bogus=bogus
  unknown flag `bogus'
  Use btcd -h to show usage

ok @jrick
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.