Update gem description

Update the gem description to match that of the Github repository
description. Also, this reads better than the original.

Change-Id: I419c829aca54eb2c930778ea9e9b87f91dfb3d24
Reviewed-on: https://gerrit.causes.com/28951
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Fix ZeroUnit linter for properties with list values

The `ZeroUnit` linter would not report lints for properties with lists
of values, e.g.

    margin: 0em 5em 2em 0em;

This was due to the fact that the regex was coded assuming a property
could only have a single value.

This has been changed to adopt a regex that scans for all instances of 0
followed by units and reports each one as a lint.

Furthermore, the linter would also not detect zero values with units in
arbitrary Sass script, e.g. `some-function(0em, 1em, 2m)`. This has been
fixed by adding a `visit_script_number` method which is called on all
numbers in Sass script parse trees.

It also changes the message reported to be more informative (using the
actual offending value in the description rather than a generic
message).

Fixes #37

Change-Id: I845309f3a2dadc79cb6d6b8806b00c4daf0ab937
Reviewed-on: https://gerrit.causes.com/28952
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>