* Attempt at clarifying and aligning unit / building loader code

Since I guess we all agree that this code is quite special, I am
interested in honest opinions on whether you think the changes in
this commit help understanding it or are even more confusing.
* No longer inline do_load, rename to _do_load

Also got rid of .iterkeys() and renamed a variable
for more consistency between unit and building code.
* Removed some if parens

These in my opinion did not improve readability. Others do and are still in.
* Remove a try/except block that never triggered

Became obsolete around the time we merged the component branch.
* "except E as err" instead of "except E, err". Refs #1191

Gets rid of syntax ambiguity: except (E, err)
Follow-up of d111f2ca90fc5d3115537a9d60bfce7f9d5f335f
* Un-escape some characters

We do not consistently use either " or ' as string delimiter anyways, so
why not switch over to the one that makes a certain string look better?
* New logbook subwidget: [BoldLabel, text goes here]

Since pychan has no support for inline text markup, this is as much
as we can do at the moment (regular text is in italics already).
Replace point.x, point.y attributes of message object by point.
Update horizons/gui/widgets/messagewidget.py

Fix erroneously changed save code in last commit.
Adapt all message to fit with the new point attribute.
Update horizons/gui/widgets/messagewidget.py

Fixed last commit,

forgot to remove a ","
* Cut back messagewidget sound_file parameter to a bool

It was only ever used as bool in the current code anyways.

The only interesting use case i could imagine (add_custom) did not use
this interface before, so if we want to have this again at some point,
that would be a suitable location to implement custom sounds.
* Indentation, removed line continuation
* Catch invalid LANG environment variables on OS X

Previously, getdefaultlocale() could trigger
  "ValueError: unknown locale: UTF-8"
A fix on the user side seems to be this:
In the "advanced" tab in Terminal's preferences,
uncheck "Set LANG environment variable on startup"
* Catch invalid LC_TIME environment variables on OS X

For some reason yet to be investigated, a locale.Error
would be raised under certain circumstances. Since the
formatting is not critical, we can omit setlocale() here.
New main square upgrade for tier settlers

The preview image is a bit smaller than the preview
for pioneers/sailors because those are not as tall.
* Add Viktoria S. to our credits

Welcome to the team!
* Use YAML anchors in tutorial as proof of concept for #1573

Every anchored argument in "logbook-data" (which is an arbitrary and
currently unused name) is also available in the parsed "events" output.

An obvious drawback of this approach is that logbook-data needs to be
defined before the actual scenario logic description can happen.
Since this cannot work with anchors, an implementation that puts the
string database below the scenario logic would most probably require
additional code on our side. For now, this should work though.
* Clean up two loops

Should perhaps change the used data structure in savegamemanager.
Also removed one check before sorting a list that is not expected to
grow very large. Could always re-introduce it if this is a problem.
Merge pull request #78 from unknown-horizons/init_clarify

* Attempt at clarifying and aligning unit / building loader code