Modified docs to export CPLUS_INCLUDE_PATH, LD_LIBRARY_PATH and LIBRARY_PATH...

Modified docs to export CPLUS_INCLUDE_PATH, LD_LIBRARY_PATH and LIBRARY_PATH correctly even if they weren't originally assigned

README.rst

@@ -23,11 +23,11 @@ Quick install for debian/ubuntu like linux distributions.
     $ mkdir build && cd build
     $ cmake ..
     $ make
-    $ export CPLUS_INCLUDE_PATH=${CPLUS_INCLUDE_PATH}:`pwd`/../include
-    $ export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:`pwd`
-    $ export LIBRARY_PATH=${LIBRARY_PATH}:`pwd`
+    $ cd ..
+    $ export CPLUS_INCLUDE_PATH=${CPLUS_INCLUDE_PATH}${CPLUS_INCLUDE_PATH:+:}`pwd`/include/
+    $ export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}${LD_LIBRARY_PATH:+:}`pwd`/build/
+    $ export LIBRARY_PATH=${LIBRARY_PATH}${LIBRARY_PATH:+:}`pwd`/build/
 
-    $ cd ../
     $ apt-get install python-virtualenv python-dev
     $ virtualenv pyrocks_test
     $ cd pyrocks_test