cmake dependency refactoring / windows build

### This will significantly change the build system

**changes**
* removed ./windows directory
* it is not possible to generate *msvc 2013* project from cmake
* cmake files refactored
  * cmake policies warnings are suppressed
  * removed EthDeprecatedDependencies.cmake
  * unified finding *boost* && *qt* libraries across all platforms
  * cmake script for finding XXX librariy moved to FindXXX.cmake
* it is now possible to download precompiled dependencies on windows
* *json-rpc-cpp* version upgraded *0.2.1 -> 0.3.2*
* removed mingw support on windows
* minimum cmake version upgraded to *2.8.12*
* fixed [static initializtion order fiasco](http://www.parashift.com/c++-faq/static-init-order.html)

**not yet finished**
* on windows, precompiled dependencies are built only in *win32* mode
* msvc project generated from cmake is only compiling in *'release'* mode.
* mscv does not copy required dll's to install directory
* downloading precompiled dependencies on other platforms (unix/mac), if required.
* cmake scripts compiling external dependencies from scratch on windows, (it would be usefull for upgrading external dependencies)

**possible future improvements**
* upgrade *json-rpc-cpp* to *0.4.1* (if required)
* mingw support
* support for older msvc versions (not only 2013)
* support for older WindowsSDK versions (not only 8.1)
* windows application installer

**questions**
* can *libethereumx*, *libpyserpent* be removed?
* can *QmlEthereum.h/cpp* be removed?
* can *walleth*, *iethxi* be removed?

alethzero/CMakeLists.txt, line 23:
The if/else condition can be removed now that eth_add_executable is used.