Clarify use of "percentage" in script interfaces

Previously, the word percentage was used (suggesting a number between 0 and
100), even when a float between 0 and 1 was actually required.

Fixes #1630
Merge branch 'master' of https://github.com/biocore/qiime into issue-1630
Merge pull request #1631 from adamrp/issue-1630

Clarify use of "percentage" in script interfaces