Added group property to field info response.

The group property can be used by a generic search application to group
related fields in the interface.
Complex field properties only report sensitivity setting now.

Before, they would list all the sensitivity alternatives, but this was
not very useful and potentially confusing. Currently supported
sensitivity settings are: ONLY_SENSITIVE, ONLY_INSENSITIVE,
SENSITIVE_AND_INSENSITIVE or CASE_AND_DIACRITICS_SEPARATE. Additional
SensitivitySettings could be added in the future, if needed.
Reported BlackLab build time of server and each index.

The server info page shows the build time of the currently running
BlackLab JAR. The index structure page shows the build time of the
BlackLab JAR used to create the index.