Add HTTP measurement analysis code

[ben-jones]

Add code to process HTTP measurements and determine if censorship is happening.
For each page, analysis will check:

• checking the page size against a control measurement (see http://conferences2.sigcomm.org/imc/2014/papers/p299.pdf)
• running regexs for known block page servers
• check for RST packets
• check for duplicate packets (this may not be meaningful, but will add an analysis flag anyway)
• check for any response packets (will show if packets are being blocked)
• check for changes in TTL values (might show proxy server or manipulation)

For each country, our analysis will check:

• do we see the same page for different URLs?
• do we see the same page length for different URLs?
• does the page size change across time? Is this consistent with control measurement?

@rpanah, @phillipa, @jakubd, @feamster, what else should we check on HTTP content?

[ben-jones]

Just thought of some more bits:

• if any headers have the same values in the control measurement, check for consistency
• check capitalization consistency with control measurement, if appropriate
• check order of headers vs control measurement, if appropriate
• check for added or removed headers from control measurement, if appropriate