some more usage examples

while experimenting and learning to use the bitcoinj library I found it interesting to have more examples. 

this PR adds examples for:
* using the WalletAppKit
* backup a wallet to a mnemonic sentence
* restoring a wallet from a seed
* sending coins
* implementing a WalletEventListener

I'm rather new to the library, if this PR is interesting it would be great if somebody can confirm the examples show the intended usage of bitcoinj :)