Added basic captcha, not hooked up
Modified matrixService.register to specify if captcha results should be sent with the registration request. This is toggleable via useCaptcha in register-controller.
Added webclient config.js for storing recaptcha public key.
Added a captcha config to the HS, to enable registration captcha checking and for the recaptcha private key.
Captchas now work on registration. Missing x-forwarded-for config arg support. Missing reloading a new captcha on the web client / displaying a sensible error message.
Add config opion for XFF headers when performing ReCaptcha auth.
80 chars please
Reload captchas when they fail. Cleanup on success.
Added instructions for setting up captcha in an obviously named file.
Added captcha support on both the HS and web client.

Merge branch 'captcha' of github.com:matrix-org/synapse into develop