Adding some developer - friendly options for testing.

Getting more into GNU oriented makefiles, Makefile.am has been reworked for
making running single tests easier. As a practical example:

    make just ample/vector.cc
    make just ample/vector.cc ample/charseq.cc ppmpf/tuple.cc
    ...

The code above will make and run tests for ample and ppmpf located in the
tests subfolder of the odreex package root. This has been done through some
clever rewiring in Makefile.am's MAKECMDGOALS. Notice that any call to make
without the "just" target being called first, will only run the tests implied
if and only if the related binaries are already compiled and present in their
tests subdirectories.