Adding unconfirmed blocks to wallet

First attempt addressing #610, still missing a detection for the actual
confirmation limit required for each block in case one lowers it in the
config.
Adding network_confirmations to configuration

* Added new configuration option `$config['network_confirmations']` to
  dist config
* Default to 120 if not set

This will allow us to define a different confirmation level for user
transactions but still display blocks at their approriate unconfirmed
value when displaying them in the wallet information for admins. Risky
for pool owners but still a viable option.

Fixes #610
Merge pull request #628 from TheSerapher/issue-610

Issue 610