Impl. 4 hour limit on retarget & adjusted getnetworkhashps

Forces retarget if no retarget for four hours
Adjusted block count in getnetworkhashps to match Noirbits' retarget
interval (60 instead of 2016)
Fixed typo (wrong comparaison operator)
Improved getnetworkhashps algorithm - takes diff. changes into account
New RPC method (getdifffromtargetbits) & minor fixes (casts) on getnetworkhashps
Adjusted hashrate calc
Fixed invalid reference
Small fix
Fixed network hashes algo - missing multiplication resulted in invalid hashrates
Added walletnotify option
Added alertnotify option

Note : neither alertnotify nor walletnotify are tested yet.
Small fix for previous commit\nInvalid code block for alertnotify
Fix bug in previous commit

Previous commit contained invalid code block (alertnotify)
Fix bug in previous commit

Previous commit contained invalid code block (alertnotify)
Merge branch 'Testing'

Conflicts:
	src/bitcoinrpc.cpp
	src/main.cpp
Merge pull request #9 from ftcminer/master

Bug fixes and extra features