MODE-2061 FileSystemConnector now uses 'openssl' if available

The file system connector now automatically tries to use 'openssl' to
compute the SHA-1 hash of files. If it is available and it returns
the same SHA-1 as streaming for a small test, then it is used
for all files larger than the 'largeFileSize' property. Note that
forking off a new 'openssl' process for small files is actually more
expensive; thus the new 'largeFileSize' property that defaults to 50kB.

The connector will upon intialization attempt to use 'openssl', and
if it is not available it will log warning messages that streaming
will be used for all SHA-1 computations. Note that 'openssl' must
be on the path for the environment in which the ModeShape process is
run.