Fix the names of the join styles.

'miter' was spelled incorrectly ('milter'), and it was swapped with 'bevel'.
Join style 0 is 'miter', and join style 2 is 'bevel'.

See, for example,
    http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/display/JointStyle.html
or
    http://infohost.nmt.edu/tcc/help/pubs/tkinter/web/cap-join-styles.html
for examples of the 'miter' and 'bevel' join styles.
Merge pull request #111 from WarrenWeckesser/fix-join-style-names

Fix the names of the join styles.