v1.7.0

Highlights:

• Documentation fixes
• Fix a bug in the export command where it always recursed
• Windows now have a show_cursor property

Changes between v1.6.1 and v1.7.0:
36bb9b5 experimental google ads support
74efd57 allow controlling ads from lua code
ddae04f iap (untested)
f158472 set transaction observer before running lua
daf8073 only show ads if filled
298323f MurmurHash: Merge MurmurHash3.cpp to latest release
526d546 Merge pull request #103 from CTXz/master
738d6c0 add window.show_cursor
51eeb6d fix error in docs (load_buffer should have been load_audio)
57933ad load_{buffer,string,script,audio,image} now return nil on error
e9d650d Merge branch 'master' into google-ads
6c564b8 game center fix
ea5c138 animate ads
686570b add iap_product_local_price
408c22f only build with google ads if framework present
d6894d4 document highdpi window setting
7b60037 ios: set audio category before activating audio
4bb563d perf_stats now includes draw and use_program calls
b3be510 fix export bug where it always recursed
e48b2e6 avoid generating sprites for characters not in font