introduce filterFragments and plotCumulativeNumberOfFragments

Dear Laurent,

this PR add `filterFragments` and `plotCumulativeNumberOfFragments`. Both have a `what` argument to choose the correct dataset, e.g. 
```s
plotCumulativeNumberOfFragments(synapterobject, what = "spectrum.ident")
```

![cnf](https://cloud.githubusercontent.com/assets/1828443/2921474/d01090c0-d6f3-11e3-8745-b744f466b0ae.png)

Additionally the `filterFragments` has a `minIntensity` and `maxNumber` argument to allow filtering by a minimal intensity threshold or a maximal number of peaks. If both are given the more aggressive one is chosen.

```s
filterFragments(synapterobject, what = "spectrum.quant", minIntensity = 100)
```

@pavel-shliaha please use `install_github("lgatto/synapter")` to get the latest version.

Best wishes,

Sebastian