use python2,

in case of running in env where python3 is default, specify python2-config (both 2.6, 2.7 are OK)

Conflicts:
	build_system/unix/configure.ac
avoid repeting paths in $PATH when running env.sh multiple times

otherwise, new paths are concateted to the old ones

Conflicts:
	env.sh
document choice of TP implementation

TP10X2 is fast,optimized implementation using C++ code
TP is python, simple, slow

Conflicts:
	examples/tp/hello_tp.py
add python2-config to Linux platform

to ntaplatform_python_libs to keep consistency with Mac