add removePrecursor/tolerance argument to load{SpectrumXml,FragmentCsv}Files

closes #59 

this PR adds the arguments `removePrecursor` (default: `TRUE`) and `tolerance` (default: `25e-6` == 25 ppm) to `loadSpectrumXmlFiles` and `loadFragmentCsvFiles`.

before:
![precursor](https://cloud.githubusercontent.com/assets/1828443/2844409/56614ef0-d08a-11e3-9b33-cc63056430d0.png)


after:
![cxdetails1](https://cloud.githubusercontent.com/assets/1828443/2844395/299a0a56-d08a-11e3-9172-6efb7e652f1f.png)

Please note I already merged this branch into the `IL` branch. That's why this branch should be merged to `master` first.