fix bash operator

=~ is an undocumented bash comparison operator, use == instead.
use TMPDIR for temporal build directory

using /tmp or equivalent can be faster and safer for this purpose.
update readme too.
build: fix variable /tmp

used only once, so make hard-coded
as in comment
build: avoid (potentialy dangerous) rm -rf on $NTA

As the $NTA is supposted to be user specified (messed up with), do NOT
remove all data in this path.
Rather print a warning.
Just lost a night worth of local commits..so that's why.
Merge branch 'mmm-fixes' of https://github.com/breznak/nupic into mmm-fixes

just me messing with git after the $NTA rm bug :P
please ignore.

Conflicts:
	build.sh