bitcoin: Update from 0.8.6 -> 0.9.1

Changes:

* The download URL also seems to have changed
* The source tarball is now actually inside the downloaded tarball
* The build system changed from qmake to autotools
* Added 
* Bitcoin now requires db48 during ./configure in order to keep wallet compatibility between systems. NixOS was using db53 before, so **I'm not sure what effect this will have on bitcoin wallets of existing NixOS systems... they may even become unreadable**
* Fixed boost detection in ./configure by providing direct path to derivation
* I also had to fix a bug in qrencode which caused a build failure in bitcoin, see bitcoin/bitcoin#3307 for details
* Enabled tests
* Worked around one of the tests needing to write to $HOME (which fails in chroot builds)
* Enabled parallel building