Fix error with --clean and relative paths
Move GPG signing to function
Move checksums generation to function
Move build dir removal to function
Force generation of GPG signature file
PEP8 coding guidelines, whitespace
buildrelease.py: enhancements and bug fixes
More issues with relative paths

Fixes problem of release files not available in the target directory at
the end of the process when doing a fresh clone in an unspecified
directory (ie. when the script creates a temporary clone).
Use check_call() instead of call() for rsync

Throws an exception in case the copy fails