fixup 7e39cbc3a5ff8ae0ab585b106564f359f249b042

whiteboard->allow_end_turn become unnegated... resulting that the
user cannot end the turn
add an operator bool() to unit_map::iterator

Semantically equivalent to `unit_map::iterator::valid()`
Uses safe bool idiom when c++11 is not enabled
move "set_menu_item" to be implemented via lua rather than C++
mouse_events: use unit_map::iterator::operator bool to simplify

the new syntax allows to greatly improve readability by eliminating
many cumbersome expressions which grab the unit_map somehow and
compare with ::end()
Use delete_file() to delete reg files in config_cache::delete_cache_files()

The reason is that, although both implementations of delete_directory()
can delete a regular file just fine, the Boost.filesystem implementation
depends upon a noisier implementation of get_files_in_dir(), which sends
an error message to stderr if the target is not a directory:

  20141223 21:54:36 error filesystem: Failed to open directory /home/shadowm/.wesnoth-1.13/cache/cache-v1.13.0-dev (cd02236-Modified)-aec97a1c3d958e534a2c379e200f91293932195b.gz: Not a directory
  20141223 21:54:36 error filesystem: Failed to open directory /home/shadowm/.wesnoth-1.13/cache/cache-v1.13.0-dev (cd02236-Modified)-b1a7724a622c9a107d2e5b70701d75e60db6ae4d.checksum.gz: Not a directory
  20141223 21:54:36 error filesystem: Failed to open directory /home/shadowm/.wesnoth-1.13/cache/cache-v1.13.0-dev (cd02236-Modified)-b1a7724a622c9a107d2e5b70701d75e60db6ae4d.gz: Not a directory

I really don't want to overcomplicate my code to deal with files and
directories separately, so I'm just sticking a check that incurs in an
extra stat() call for every entry to choose whether to use
delete_directory() or delete_file(). This is all very infrequently-run
code anyway, and modern OSes are smart.
fs: Quietly ignore non-dir targets in get_files_in_dir() (BFS only)

The non-BFS implementation already ignored this case without making
noise in stderr, and this function is used by delete_directory(), which
also accepts both regular files and directories. The add-ons management
code has always relied on this behavior.
Revert "Use delete_file() to delete reg files in config_cache::delete_cache_files()"

This reverts commit d06c39b714ef48e5e6dc96bc62a73c994f6e8c45.
fix compilation after 38f5aaa56ef87ed58ae03239ac0a600988c0fd09
move the place, remove shroud WML tags to lua implementation
move allow_undo, allow_end_turn impl's to lua
move clear_menu_item to lua implementation
add time_area registration impl to lua
move replace_schedule impl to lua
travis: refactor, add a clang C++11 build

merged build types 1 and 2
travis: disable a warning for clang c++11 build
move WML [scroll] implementation to lua
travis: fixup $CXX check
travis: fixup HEAD^ (conjunctions)
fixup wml menu items: track the lifetime of the manager

testing revealed that assertions may fail regarding the lifetime
of the manager of wml menu items in some cases. this commit adds
a pointer to each which tracks its manager's life time and prevents
dangling pointers.
travis: replace build numbers with descriptive names