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may not be a big deal in such a simple case, but just as suggested in review.

We don't have to worry about the case lock() throws in the constructor.
In that case the destructor won't be called.
This simplification will make the code simpler, and we can also make
mutex_ member variable a const.

(maybe a matter of taste, though)

So we don't throw from a destructor. This should never happen anyway.

They are not needed currently.

* Documentation
* Exception texts
* Exception safety for a rare situation

* Removed an unneeded friend declaration.
* Typo

- spacing
- new lines for readability
- const
- brace position for catch
- position of ++/--

It should be the same according to the standard, but Clang seems not to
agree with that.

It can be used to check the thing is locked.

Created with minimalistic set of functionality. Created a new library
for it, there'll be more thread-related stuff here.