Ensure that all the necessary cleanup is performed in case the allocation fails
with an exception - files are closed, buffers are reclaimed, etc.

Any test that triggers a simulated out-of-memory condition is ran once again
with a throwing allocation function. Unobserved std::bad_alloc count as test
failures and require CHECK_ALLOC_FAIL macro.

Fixes #17.

Also add tests that verify save_file for absence of FILE leaks.

Previously attributes that were copied with their node used string sharing,
but standalone attributes that were copied using xml_node::*_copy(xml_attribute)
were not.

If an out of memory error happens in load_file there's a danger of leaking
the FILE object. Since there is a limited supply of the objects we can easily
test that the leak does not happen.

Instead of reallocating the string for every tree level just do two passes
over the ancestor chain.

as_utf8_end was used with std::string, where writing an extra zero-terminating
character should *probably* always work (at least if size is positive) but is
not ideal.

The only place that needed to zero-terminate was convert_path_heap.

Previously there was no guarantee that the tests that check for out of memory
handling behavior are actually correct - e.g. that they correctly simulate out
of memory conditions.

Now every simulated out of memory condition has to be "guarded" using
CHECK_ALLOC_FAIL. It makes sure that every piece of code that is supposed to
cause out-of-memory does so, and that no other code runs out of memory
unnoticed.

We currently don't allocate/modify any state so there are no issues with this.

This makes sure that no exception handling mechanisms are used if
PUGXML_NO_EXCEPTIONS is defined.

When parsing XPath variables, we need to perform a heap allocation; if it
fails, an xpath_exception instead of bad_alloc used to be thrown.

Now we throw the exception of a correct type so that xpath_exception means
'parsing error'.

This provides more coverage for #17.

Base directory is now using target basename.

This eliminates one more hardcoded version from the repo, yay!

It's almost done; the only remaining issue is that some section titles are too long.

Also minor wording fixes.

Users no longer need to #include "pugixml.cpp"

Mostly added correct quotation to changelog.

This is mostly done using regex replaces of original Quickbook markup, plus a
bit of manual fixup for multiple references to the single point from different
lines that AsciiDoc does not seem to handle.

Still need to replace [link ] with actual links.
Also a bunch of small fixes here and there.

Also remove redundant [lbr]