filetools.py: pep8 clean up
checksums: read sha1 and md5 into blocks

When calculating the checksums of a large file, we need to read it into
blocks (currently 16MB) to keep memory usage acceptable.
filetools.py: fix indenting
filetools.py: fix typo
filetools.py: use python 2.x stuff
filetools.py: python 2.4 comp. => don't use blocksize
filetools.py: nicier imports of checksums and explain magic number
filetools.py: use hashlib like wrapper for addler32 and crc32
filetools.py: small changes after remarks
filetools.py: reshuffled a bit
ZlibChecksum: use new style class
filetools.py: change some variable names

Represents more correct what the variable is/does.
Merge pull request #836 from wpoely86/checksums

Checksums: use blocks to read the files