Add sanity checks to parse_coords ( fixes #982 )

Add a couple checks to the coordinates parser, doing this prevents the
parser to load distance matrices seamlessly. Specifically the function
will now check for:
    - First line being the number of vectors.
    - Second to last line being eigenvalues.
    - Last line being the percentage of variation explained.

When these conditions are not mete a `QiimeParseError` exception is
raised; the test suite was extended to check it raises these errors on
different cases.
Remove extra percent sign in coordinates string

This percent sign would have only be needed if the string was being
formatted.
Change exception formatting and if statements

Formatting the exception messages like this will make the transition to
py3k easier.
Merge pull request #1178 from ElDeveloper/issue_982

Add sanity checks to parse_coords ( fixes #982 )