Added getHeaderSession to get session from servant header

[mschristiansen]

I've updated the example to Fix #30 also added a getHeaderSession to the library and factored out shared code from getSession.

Let me know if there is anything you would like me to change or isn't happy with.

[zohl]

Seems good, except there are few failures in the travis:

• for servant-0.8 there should be imported toHeader method.
• for earlier versions of servant there is no such method to import at all (i.e. your changes will not work with those versions unless there is a workaround, which might not exist).

If you're using servant-0.9 or higher, this is not a big deal. You can just wrap the code in CPP-directives, so that for older versions of servant there will be old code, and for newer versions there will be new (yours) code. (See directives with condition MIN_VERSION_servant(0,9,0) in Servant.hs for example).

[mschristiansen]

I've added CPP-directives. Fingers-crossed that all the tests pass this time.

[mschristiansen]

Installing deps with Nix and testing all build options locally. Should probably have done from the beginning.

[zohl]

Btw, if you have any problems with testing locally, do not hesitate to update the PR and check travis builds :) The one with parameters CABALVER=1.18 GHCVER=7.8.4 won't build anyway (it was like that before), the one in "allowed failures" might fail too. But the rest should pass successfully.

[mschristiansen]

Ok, thanks. Will do that. Will ping you again once it's ready, so don't worry about it until then. Just have some actual work to do first (it suddenly become urgent).

[mschristiansen]

@zohl Tests are passing. Tried to limit the number of CPP statements, but not great.

[zohl]

Awesome, good job! Thank you!