Theme as contextmanager

New try to get theming in a context manager

New in this PR:
* rcParams are now added one by one to get these params validated
* adding rcParams is now wraped in a try:...except: to get a better error message when something is wrong
* the default rcParams are unchanged to the old theming (string vs int/tupple) and the matplotlib internal validation during setting them is convertig them to the real values

Old:
* use a context manager during plotting (Closes: #42, #64)
* set default values, no more rc files (closes: #15, #7)
* removed the MacOSx backend (Closes: #23)