core: add guards to m_rsc_gone functions.

This gives a better error message.

Fix for controller_page:previously_existed/2 to return 404 for unknown resource id's.

Fixes #459.

Also an additional fix for controller_page:resource_exists/2,
as discussed in commit 45bca641 on github.
doc: describe options for the lib tag.

Support for these options were added in 24116b0a and b534f9ac.
core: add warning on missing lib files.

Issue a warning if a file mentionend in a {% lib %} tag is not found by the module indexer.
mod_base: add soft jquery-latest links to ease upgrades.
mod_admin: flush cached admin menu on module activate/deactivate.
mod_development: Use the inotifywait command found with the "which" call
gen_smtp: New submodule

For this commit:
https://github.com/Vagabond/gen_smtp/pull/31
mod_development: Fix false positive in inotifywait finding
core: install additional modules listed in site config.

Since there may be additional modules needed by a site during install,
it is now possible to list them in the site config with a 'modules' key,
when using the 'empty' skeleton:

[
        ...
        {skeleton, empty},
        {modules, [mod_foo, mod_bar]},
        ...
].
core: optional: install default menu from site config.

When using the empty skeleton, look for a default menu in the site config:

[
        ...
        {skeleton, empty},
        {default_menu, [{menu_item1, []}, {menu_item2, [{submenu_item, []}]}]},
        ...
].
core: move skeleton modules and menu to site config.
doc: Fix typo in startup script example
doc: Document the "redirect" site config option.
doc: Document site's streamhost, smtphost, cookie_domain options.
doc: Fix another bug in startup script
core: Fix spec for z_install_data:install/2
doc: Make 'make stubs' more portable

Do not hardcode path to sed; just use 'sed' in the path.

http://unix.stackexchange.com/questions/14887/the-way-to-use-usr-bin-env-sed-f-in-shebang
core: Fix deletion of resources.

@kaos broke it in ae823f4a82e5529c73593f8a7c80ba96771c72fc
core: Fix typo in blog skel config.in
mod_admin: Added rebuilt Bootstrap, from our own fork

See: https://github.com/zotonic/bootstrap/commits/zotonic-admin

Fixes #456