use python2,

in case of running in env where python3 is default, specify python2-config (both 2.6, 2.7 are OK)
avoid repeting paths in $PATH when running env.sh multiple times

otherwise, new paths are concateted to the old ones
document choice of TP implementation

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

Conflicts:
	examples/tp/hello_tp.py
Merge remote-tracking branch 'breznak/minor-improvement-python-ver' into greg-python27-support-modular-numpy
Removing the --ignore-installed CLI option.  When present, numpy will always
re-compile/re-install which is not the desired behavior and by not specifying
it on the command line, the user can always override with either a pip config
file in $HOME, or by PIP_IGNORE_INSTALLED env variable.

Ultimately, we'll be getting rid of --find-links, --no-index, and --index-url
options too, so it makes sense to keep the pip command as light as possible.