[trivial] Fix three recently introduced typos

[trivial] Fix three recently introduced typos

```
$ git blame src/policy/fees.cpp | grep becuase
3810e976 (2017-03-07 11:33:44 -0500 789)          * checks for 2*target becuase we are taking the max over all time
$ git blame src/policy/fees.h | grep successfullly
2d2e1705 (2017-04-12 12:29:03 -0400  54)  * representing that a tx was successfullly confirmed in less than or equal to
$ git blame src/wallet/feebumper.cpp | grep "hasen't"
a3878374 (2017-05-11 09:34:39 +0200 258)     // make sure the transaction still has no descendants and hasen't been mined in the meantime
```