Syncronize with 0.4.4.6 release

Changes list:

* Add checkpoints;
* Remove stake weight switch code;
* Remove output limit for testnet since publishing / for mainnet since 1 May 2014;
* Remove mandatory fee for testnet since publishing / for mainnet since 1 may 2014.
 Compatibility improvements

* Add a generic SALSA implementation and disable optimized implementations by default;
* Remove macroses from ARM SALSA implementation;
* Some misc defigition fixs to correct mingw-w64 compatibility issues.