Merge pull request #13 from kmike/json-render-endpoint2

render.json endpoint
bugfix: defaults.WAIT_TIME was not actually taken into account
full page screenshots (as discussed in GH-14)
require non-zero 'wait' if viewport=full
tests for viewport=full
defaults.VIEWPORT can be set to 'full'
README notes about relationship between viewport=full and wait=0
Merge pull request #19 from kmike/full-page-screenshots

[MRG] full page screenshots (as discussed in GH-14)
Merge branch 'master' into proxy-support