Make JVM initialisation variables configurable via environment variables.

[carlwilson]

There are several issues, such as #115, that can be caused by the JVM running out of heap space. Many of these could be addressed by passing parameters to the JVM to increase the initial allocation or limits.

It should be possible to use a combination of environment and script variables to set default values and override them from the shell. For the *nix shell script a solution might look like:

# Set XMX value to JHOVE_THREAD_STACK environment var or default.
XMX_VAL="${JHOVE_THREAD_STACK:-1024k}"

# Set XSS value to JHOVE_HEAP_MAX environment var or default.
XSS_VAL="${JHOVE_HEAP_MAX:-64M}"

# Set class path and invoke Java
java -Xss"${XSS_VAL}" -Xmx"${XMX_VAL}" -classpath "${CP}" edu.harvard.hul.ois.jhove.Jhove -c "${CONFIG}" "${@}"