Fix publish.py IPython import Error and make Python3 ready

Fix Error when importing pymatbridge and IPython is not installed.
Change setup.py and __init__.py to make pymatbridge Python3 ready.
Revert "Fix publish.py IPython import Error and make Python3 ready"

This reverts commit 5bcd78c5191d5b74d09e7a87021eb4fcaed9b929.
Fix #60: publish.py fails if IPython not installed

This fix just surrounds the import statement in __init__.py with try
except. This prevents publish.py from being imported of IPython is not
present.
Python3 compatibility

This commit fixes two compatibility issues. In setup.py execfile was
changed to support Python3 and in __init__.py the imports are now
relativ to support the changes made to the import system in Python3.
Fix readme

One space was missing on line 86
Merge pull request #61 from Mailerdaimon/master

Fix #60 and add Python3 compatibility