xeth: fix #640, panic converting nil recipient to hex.

Fetching the recipient address from a transaction was changed to return nil
instead of a zero-address, but this code path was not updated, so whenever
a contract was created, a nil panic occured.

bugfix as a result of PR 671

@zelig: Temporarily commented out TD check untill the rest of the network has
been fixed.

Merge branch 'frontier/blockpool' of https://github.com/ethersphere/go-ethereum into ethersphere-frontier/blockpool

- add peer switch channel arg to activateChain - no peer locking within
- proper locking in AddBlock - fixes deadlock issue
- comment out TD check and skip incorrect TD test again for hotfix

- queued       bool // flag for blockpool to skip TD check
- set to true when future block queued
- in checkTD: skip check if queued
- TODO: add test (insertchain sets future block)

- TestPeerPromotionByTdOnBlock renamed and skipped for now test should pass iff
  if TD is updated based on an agreement
- senders register in AddBlock, flag records if they are coming from newblock
  message (and therefore advertise their TD with the block) or block message
  (TODO: latter are stored on the cache and updated by checkTD call;
  protocol should also call AddBlock on newblock messages by non-best peers)
- remove TD update from optional TD field in addBlock: this is no longer part of
  the eth protocol spec -> TODO: reflect in wiki
- only initialise peer map if at least two

- do not break from headsection on error
[remove peer after protocol quit will close switchC, until then head block can arrive and block on channel while keeping peers lock causing a deadlock.]
- more careful locking in AddBlock

Support for import/export hex encoded keys

Add path expansion support for command line arguments, closes 567

Removed the from as a requiremet from the RPC eth_call. Xeth#Call now
also default values to:

1. Supplied account
2. First account if any
3. No managed account => 000000..00

The transaction pool will now some easily be able to pre determine the
validity of a transaction by checking the following:

* Account existst
* gas limit higher than the instrinsic gas
* enough funds to pay upfront costs
* nonce check