Make --zero-cap the default for colorspace reads.

Negative quality values do not make sense and the spaces in the resulting
FASTQ file cannot be handled by many tools.

The option can be disabled with --no-zero-cap. If no quality values are
available, the option is also disabled.

This is issue #75, see https://code.google.com/p/cutadapt/issues/detail?id=75 .