Fix error message for proton mass.

Change it from charged pion into proton.
Add TVector3 to possible source variables for TLorentzVectors.

Add TVector3 to possible source variables for TLorentzVectors.
Merge pull request #12 from cdreisbach/pr2

Fix proton mass constant error message, extend getLorentzVector.
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.