Remove option to display server signature in the footer of every page.

* theme/contentfooter.php: completely remove display of information about the
server, php, and database types and versions that bumblebee is running on. Such
information is only useful for sysadmins, and exposes too much for the casual
user.
* config/bumblebee.ini: remove 'server_signature' option from default
configuration file.
* inc/bb/settings.php: remove 'server_signature' option from bumblebee's web
interface settings page.
* inc/formslib/sql.php: remove db_get_version() and db_get_name() functions
since they were only being used for the server signature.
* inc/systemstats.php: remove webserver_get_name() and webserver_get_version()
functions since they were only being used for the server signature.
inc/formslib/sql.php: add missing ')' on code.
Create 'templates' directory and 'bumblebee' template from current theme.

With the purpose of allowing alternative themes without modifying the original
source, all theme files will now create the default bumblebee template. A
new directory, 'templates', will allow for multiple templates to exist side
by side, with choice of template made at configuration level. The actual code
and configuration for this still needs to be implemented on a separate commit.
Add support for installation of multiple templates.

* config/bumblebee.ini, inc/bb/settings.php: add template option (defaulting
to 'bumblebee') and removal of emailTemplate, emailRequestTemplate,
htmlWrapperFile, and emailTemplate which are now part of the template (not a
configurable option).
* inc/actions/billing.php, inc/actions/bookcontact.php,
inc/bb/bookingentry.php, inc/bb/calendar.php, inc/bookings/booking.php,
inc/bookings/vacancy.php, inc/export/htmlexport.php, inc/export/pdfexport.php,
index.php, templates/bumblebee/contentheader.php,
templates/bumblebee/export/exporttemplate.html,
templates/bumblebee/pagefooter.php, templates/bumblebee/pageheader.php: replace
replace lookups of 'theme/...' by 'templates/$template/...'. All of this paths
are still very much hardcoded and should be made simpler if templates are to
become viable option.