cross matching performance plots

Dear Laurent,

this PR adds the cross matching performance plots we discussed yesterday. 

![performance](https://cloud.githubusercontent.com/assets/1828443/2889477/cb72bdb4-d51c-11e3-8a09-d6e64a5f5b13.png)

It also adds a `what` argument to apply `filterMatchedEMRTsUsingCommonPeaks` on `"non-unique"` or `"all"` matches.

Additionally the cross matching results are append to the `MatchedEMRTs` data.frame and could be written to a CSV file using `writeMatchedEMRTs`.

Best wishes,

Sebastian

\cc @pavel-shliaha