Minting tab

Here is my first version for the minting tab.

#### Screenshots :
![minting_tab](https://cloud.githubusercontent.com/assets/539524/3146803/40653ddc-ea4d-11e3-994c-045262ad886d.png)
![minting_tab_minting_menu](https://cloud.githubusercontent.com/assets/539524/3146804/42d96192-ea4d-11e3-9b69-ef883fc3a3fd.png)

#### About the minting probability : 
I have an ongoing discussion with Jordan about how to display this particular data. For now, i have ported the same calculations as made by fuzzybear on his calculator (http://poscalculator.peercointalk.org). I'm waiting for more precisions on his idea before going further.

#### Notes : 
* I've done my best trying not mess most of the existing code. So i had to duplicate the getDifficulty() function originaly in rpc client (https://github.com/Peerunity/Peerunity/blob/master/src/bitcoinrpc.cpp#L62) to get it available in main.h (https://github.com/daeMOn63/Peerunity/compare/Peerunity:master...master#diff-e8db9b851adc2422aadfffca88f14c91R1223). I have not yet removed the original one, waiting for your advices on how to handle this.

* I've also tried to prepare most of things to be reusable for implementing the listminting function on the rpc client. It should be quite easy to get this running now.

* Table header's tooltips may also be incorrect (see : https://github.com/daeMOn63/Peerunity/compare/Peerunity:master...master#diff-2a70fc4ced71141820496ae7f1def5c5R359 )

* I had troubles understanding the update process from the transactionTableModel. I cannot reuse the same here because transactionTableModel update a tmp vector on the wallet. I managed to get something working (see : https://github.com/daeMOn63/Peerunity/compare/Peerunity:master...master#diff-2a70fc4ced71141820496ae7f1def5c5R215 ) but it may be inapropriate / inefficient.

Maybe indicate the unit is days.