Quote cmdline call to mpileup, fixes #15
If cmdline call of samtools mpileup fails, report the error.

* Introduce systemu as a dependency. Should not be a big problem given systemu's wide usage.
* One downside is that now the entire pileup output is read into memory at once, as opposed to being a stream as before.
* Equivalent methods should probably be implemented in mpileup_plus, but the author of this patch is not familiar with that functionality"
Merge pull request #20 from wwood/quote_mpileup_cmd

Quote mpileup cmdline