add sep argument to utils.vec2ssv and Co

Dear Laurent,

in `synapter` are a lot of `data.frames` that containing stuff like: "1,2,3,4"

I want to use the `MSnbase:::utils.ssv2vec` function (I can reimplement it, because in fact it is a simple `strsplit` with some cleaning, but it has unit test etc.). That's why I add a `sep` argument (I don't understand why I didn't add it in the first implementation :confused:; Sorry!):
```s
utils.ssv2vec("1,2,3,4", sep=",")
```
You use something like this in `MSnbase`:
```s
pattern <- strsplit(pattern, "")[[1]]
```
That could be replaced by
```s
pattern <- utils.ssv2vec(pattern, "")
```
(ok, I don't know whether it is worth the effort)

I send this as PR and don't commit it directly because tomorrow is the 7th of April (and I am not convinced that we need this feature in BioC 2.14):
> April 7
*    Bioconductor release candidate. Package maintainers should limit changes to "show-stopper" bugs and documentation improvements.

> April 11
*    Creation of the BioC 2.14 release branch. Development can resume on trunk, but changes will not be part of the release.
*    Build final release repositories.

Best wishes,

Sebastian