'intensity weighted mean' averaging added to ProteinQuantifier, solves #830

This pull request adds the option of 'intensity weighted mean' to the ProteinQuantifier tool.
@hroest We confirm your results on a fixed ratio SILAC sample. I left everything in the analysis fixed apart from the averaging. Here the standard deviations of the fc distributions.

meadian    sd = 0.9087
mean    sd = 0.8646
weighted mean    sd = 0.9276
sum    sd = 0.8646

Now I am for changing the default to "sum".

![openms_sum_weighted_mean_density](https://cloud.githubusercontent.com/assets/6149805/5057948/4e0c1dbe-6cdb-11e4-8b50-ce64422cd373.png)