Improve documentation

- Minor fixes, such as missing periods, capital letters and typos.
- Add Travis CI badge.
- More informational descriptions of some options.
- Strongly recommend the `source` option of `css.parse`, and make the
  example proper by using it.
Remove `options.filename` from `css.stringify`

Why?

- It is only used to set the `file` property of the source map, and
  that’s probably done just because the `file` property used to be a
  required property of source maps. It isn’t no more.
- The `file` property is not used by browsers.
- The user could easily just add the property himself on the returned
  source map object.
- It was neither documented nor tested.
- KISS.

@conradz:

> I think if we decide to deprecate filename, we can just remove it
> altogether and bump major version.

https://github.com/reworkcss/css-stringify/pull/42#issuecomment-44884423
Make relative applied source paths relative to CWD

When generating a source map, the paths put into the `sources` array of
the source map normally come from `pos.source` properties on the AST
nodes. These are today assumed to be relative to the current working
directory, since that’s how any original source maps are resolved and
applied.

Paths in the `sources` array can also come from applied original maps.
Relative such source paths are relative to the original source map --
not to the current working directory!

It does not make sense to mix paths that are relative to different
things. Therefore this commit makes sure that applied relative source
maps are rewritten to be relative to the current working directory, like
all other sources.

An existing test was updated to test this.

The fix uses the third parameter of the `.applySourceMap()` method,
which must be a URL -- a Windows-style path does not cut it. While the
applied source paths should already be URLs (since they come from a
source map, which should be used in browsers), we call `path.dirname()`
on them, which returns Windows-style paths on Windows. Therefore `urix`
is used to convert them back to URLs again.

For consistency, I then went ahead and fixed another issue: Before
adding any sources into the source map, we should convert them to URLs
if they are Windows-style paths. That is as easy as calling `urix` on
each source first. A test has been added for this.
Merge pull request #25 from lydell/improve-docs

Improve documentation
Merge pull request #26 from lydell/remove-options.filename

Remove `options.filename` from `css.stringify`
Never add source mappings at the end of nodes

It is completely useless and only makes the source map unnecessarily
large.
Merge pull request #28 from lydell/source-paths

Make relative applied source paths relative to CWD
Merge remote-tracking branch 'lydell/no-mappings-at-end'

Conflicts:
	lib/stringify/source-map-support.js
Update History
Update history
Switch to tilde version range for source-map-resolve

It is 0.x and may introduce backwards-incompatible changes in minor
version increments.