Substitute headers

[PilchardFriendly]

There are some cases where the upstream client would like to control which headers are sent to the downstream servers.

An example of this is where prior authentication has occurred, and so unique Authorisation headers must be sent separately to primary, secondary and candidate servers.

In the PR, I've designed it so that, for HTTP(S) protocols only, headers will be rewritten (or dropped) if they are prefixed by "primary", "secondary", "candidate_".

For instance, the header "primary_Authorization: Bearer ajdskhasqqjkh19qw1h2" will be sent to only primary, as "Authorization: Bearer ajdskhasqqjkh19qw1h2".

This maintains diffy's stateless communications, but supports testing through Postman, etc.

[PilchardFriendly]

Travis CI seems to have a borked setup - builds pass on 4/6 environments.

[PilchardFriendly]

Looks like something wrong with java 1.7.0_91 - this has been EOLd on 25 Feb 2015. Is it still necessary? If so - we can have a look at what is causing Travis to coredump.

[codecov-io]

Current coverage is 37.51%

Merging #46 into master will increase coverage by 1.01%

@@             master        #46   diff @@
==========================================
  Files            29         31     +2   
  Lines           789        821    +32   
  Methods         772        801    +29   
  Messages          0          0          
  Branches         17         20     +3   
==========================================
+ Hits            288        308    +20   
- Misses          501        513    +12   
  Partials          0          0          

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