added install/uninstall

This adds install and uninstall to the Makefile of the Unix port.
micropython is installed to /usr/local/bin
uninstalling removes the executable  from there

Note that this is the first time ever I made changes to a Makefile. It tested on Ubuntu and it seems to work fine.

```
plam@newlinx:~/Documents/micropython/unix$ make
mkdir -p build/genhdr
CPP ../py/qstrdefs.h
makeqstrdata ../py/qstrdefs.h qstrdefsport.h
Generating build/genhdr/py-version.h
mkdir -p build/py
mkdir -p build/py/../extmod
CC ../py/nlrx86.S
.
.
.
CC modffi.c
LINK micropython
   text	   data	    bss	    dec	    hex	filename
 290151	   1384	   1400	 292935	  47847	micropython
plam@newlinx:~/Documents/micropython/unix$ sudo make install
Use make V=1 or set BUILD_VERBOSE in your environment to increase build verbosity.
install -D micropython /usr/local/bin/micropython
plam@newlinx:~/Documents/micropython/unix$ micropython
Micro Python v1.2-99-g2eeeafc on 2014-08-11; linux version
>>> 
plam@newlinx:~/Documents/micropython/unix$ cd ..
plam@newlinx:~/Documents/micropython$ micropython
Micro Python v1.2-99-g2eeeafc on 2014-08-11; linux version
>>> plam@newlinx:~/Documents/micropython$ 
```