One executable per Catch-based test

As discussed in #35, refactor Catch-based tests so that an
executable is compiled per each test file.

This way the output of `make check` is less surprising since
`7 tests have passed` as opposed to `1 test has passed`.
Update .gitignore to ignore more stuff
test: rename buffer_tests.cpp to buffer.cpp

No other test has the `_test` suffix. In the interest of uniformity,
so let's give to the buffer's test program a better name.
test/common/bufferevent.cpp: remove uneeded include
test: batch of cosmetic changes

More spaces around includes. Also add three line explanatory comment
to a file that was missing such comment.
Merge pull request #36 from bassosimone/feature/refactor_catch_based_tests

Refactor Catch-based tests: one executable per test
Add the boost modules needed by yaml-cpp
Add infrastructure to generate BOOST_INCLUDEDIR

Yaml-cpp uses CMake. AFAICT, CMake needs to know the BOOST_INCLUDEDIR
in order to compile something using boost.

This diff proposes a way to generate BOOST_INCLUDEDIR, integrating with
our autotools infrastructure.

Basically, there is a Makefile.in in src/ext/boost that intercepts the
targets generated by automake and does something sensible.

In particular, when the target is "all" (meaning, "build everything"),
the Makefile.in copies the boost headers in the same place.

Similarly, the "clean" target (and other "clean-*" targets) delete all
the headers that were copied.

With this setup, I can already build yaml-cpp. More automation will
follow.
Add my fork of yaml-cpp as a submodule
Add infrastructure to compile yaml-cpp
src/ext/{boost,yaml-cpp}: improve the build infrastructure

- Remove directories that can be creared at build time.

- Deal with @builddir@ != @srcdir@, when it is easy to do that.

- Use more standard variable names.
.gitmodules: use the http url (as opposed to the ssh url)
src/ext/include.am: don't define SUBDIRS twice
ext/yaml-cpp: use absolute prefixes in Makefile.in

This should unbreak travis-ci compilation or, at least, break
it a little closer to success.
Add debugging statements to understand what's going on
Add the required boost headers
Fix yaml-cpp Makefile.in; now it should work
Make sure we pass autotools CXX and CC to cmake

Should hopefully fix compilation issues with travis-ci.
Fix building of yaml-cpp
Start outlining report classes