Fix some typos and offline tutorial
 Fix some typos in the documentation, and add a check for installed version of
 the Python tutorial. If installed version of the tutorial is found, PySpread
 will use this one instead of the online one.
 This patch also updates the link to the online Python tutorial.
Merge branch 'master' into v0.2.4

Conflicts:
	pyspread/src/gui/_main_window.py
Added functionality of commit commit 7d60c5d8d54dbc3e6e46b69c1c12e0a875260ea3 to v0.2.4