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