Add mycroft-core to virtualenv path

Add the insertion of mycroft-core in the virtualenv created by
dev_setup.sh.  It becomes part of the python2.7 site-packages.
This is needed for running mycroft-core pieces directly from
the command line, after performing "workon mycroft".

dev_setup.sh

@@ -78,6 +78,9 @@ cd "${TOP}"
 easy_install pip==7.1.2 # force version of pip
 pip install --upgrade virtualenv
 
+# Add mycroft-core to the virtualenv path
+echo "$TOP" > ${VIRTUALENV_ROOT}/lib/python2.7/site-packages/mycroft-core.pth
+
 # install requirements (except pocketsphinx)
 # removing the pip2 explicit usage here for consistency with the above use.
 pip install -r requirements.txt