Use of a library for the adjustment methods.

This pull requests pulls in the pull requests #61, #63 and #88.

I have written [an abstraction layer called libgamma](https://github.com/maandree/libgamma) that takes care of all adjustment methods. This pull requests makes of if this library rather than implement support for all display servers in Redshift and letting Redshift's code deside in which order to try adjustment methods. I have however left in the possibility to extend Redshift with new adjustment methods directly without going through libgamma.