Prepare for next major libsass release

[mgreter]

This issue is used to track the progress toward a libsass 3.0 release.

• [x] Update to the latest implemention
• [x] Implement command line args for new options
• [x] Document new or changed features
• [x] Add documentation for source map options
• [x] Add documentation for changed comment options
• [x] Add documentation for exposed sass2scss function
• [x] Update documentation to reflect new options for sass_compile

  Pending PRs

• [x] https://github.com/sass/sassc/pull/70: sassc build changes for coverage
• [x] https://github.com/sass/libsass/pull/503: libsass build changes for coverage
• [x] https://github.com/sass/sassc/pull/69: Better sass_interface api standardization

  Still missing features

• [x] Implement new map data type
• [x] Update documentation for map type

  Change internal structure of perl sass values

Currently maps and lists do not directly map to their possible perl representation (ARRAY/HASH). Therefore I will re-work the internal representations to have maps being represented by actual hashes and lists by actual arrays. To do this, I will need to change all other data types to a reference. Strings will be scalar references, while all other types will still point to an array structure, since we need to save additional information (like the unit for numbers). Implement stringify for these data types to get the actual representation (maybe do this by invoking libsass to get the same representation). We also will make sure that we pass the same structure around for real perl and for the xs implementation.

• [x] Write tests for the proposed format
• [x] Refactor internal representation
• [x] Allow functions to return native types
• [x] Document the new internal xs structures

Will adjust the version of CSS::Sass to libsass (updating it to 3.0)

[mgreter]

Libsass 3.0 has been released and I have an implementation for sass_value data types, backed by 269 unit tests. I'm not 100% happy with the end result, as I created more complex internal data structures than I first wanted to have. Although it seems to work well, it feels a bit over engineered, but it could IMO be changed at some later point without much pain. This pretty much reworks all of the internal sass_value handling. It should now be much more intuitive to work with values from/to libsass within custom functions.

Updated a lot of documentation which could need some peer review :)

//CC @caldwell

[mgreter]

Since I got no feedback I'll relase this version "as is" to cpan!!!