Update README.md

Updated link for build instructions.
Merge pull request #186 from shobute/patch-1

Update README.md
Re-enable "getheaders" network command.
Merge pull request #188 from domob1812/enable-getheader

Re-enable "getheaders" network command.
Add CTxIndex::GetHeight routine.

Split the old code of CTxIndex::GetDepthInMainChain to allow also
GetHeight() and GetContainingBlock() to be used.
Add IsUnspendable method, blocking dust spam outputs.

Implement IsUnspendable() as additional check when verifying
transactions.  This method checks for provably unspendable outputs (will
be useful later on for the UTXO set), and it also blocks dust spam
outputs created between blocks 39k and 41k from being spent in the
future.  This enables them to be removed from the UTXO set.
Block dust spam tx from mempool before fork point.

Don't accept dust spam tx to the mempool even before the fork point.
Blocks with them are still considered valid, though.