add idSummary

Dear Laurent,

this PR introduces `idSummary`:
```s
quantFile <- dir(system.file(package = "MSnbase", dir = "extdata"),
                 full.name = TRUE, pattern = "mzXML$")
identFile <- dir(system.file(package = "MSnbase", dir = "extdata"),
                 full.name = TRUE, pattern = "mzid$")
msexp <- readMSData(quantFile, verbose = FALSE)
msexp <- addIdentificationData(msexp, filenames = identFile,
                               verbose = FALSE)
idSummary(msexp)
#   file identFile coverage
# 1    1         2      0.6
msexp <- removeNoId(msexp)
idSummary(msexp)
#   file identFile coverage
# 1    1         2        1
```
I also add a small entry in the vignette (for `removeNoId`, too).

Best wishes,

Sebastian

P.S.: Sorry for the large differences in the files because of the removal of spaces at the line endings.