build system: require yaml-cpp version 0.5.0

Does not compile with version 0.3.0 previously required, as some methods
of the YAML::Node class are missing. Note that the required version is
not checked against the version of yaml-cpp found.
build system: allow python 2.6

Modified the FindPython Cmake module to also work with python 2.6, and
changed the required version in CMakeLists.txt.
Merge pull request #14 from suhlatwork/yaml-ver

build system: require yaml-cpp version 0.5.0
Merge pull request #15 from suhlatwork/python-ver

build system: allow python 2.6.
Add additional 'iostream' include

Otherwise 'cerr' might not be known (compiler error appeared after
switching from boost 1.55 to 1.56).
Whitespaces

Correct a couple of whitespace mistakes.