v1.8-rc1

First relase candidate for v1.8. Highlights:

• improve automatic test converage: also test reverse mode (make test)
• add per-file IVs based on the inode number to reverse mode to improve security
• add automatic benchmark (make benchmark)
• compare MAC in constant time ( fixes bug #12 )
• add --nocache option
• lots of fixes to make building on OSX easier