abstracted md5 support to general check sum support to be able to add sha1 support
- moved hash functions into separate file hash.cpp/.h
- added HashFile class
- added buildsystem option for compiling sha1 support
- refactored build system
- extended packager to be able to create sha1 check sums with the '-checksum sha1' option

sha1 support is not ready, the following work is left:
- add config option to define which hash type is used in a specific release
- extend package hash value checking to sha1 sums (shared/package.cpp)
- add fallback md5 solution if sha1 sums are not available
- added sha1 support to emerge