Improve custom importer to pass parent import context

One example would be a custom importer that loads the
content from the web. If this includes another import,
it should be loaded relative to the parent file. To do
this, the custom importer needs to know the context of
the parent import. Which is what this commit will fix!
Fix default argument passing for custom c functions

In reference to https://github.com/sass/libsass/issues/708
Fix fractional output for truncated zeros
Implement @error function and Sass_Warning for C-API
Code: Adds an include statement for VS.
Merge pull request #719 from am11/master

Code: Adds an include statement for VS
Fix embedded sourcemap content type
Merge pull request #723 from xdissent/map-content-type

Fix embedded sourcemap content type
Unquote keyword arg names
Merge pull request #725 from xzyfer/fix/quoted-keyword-args

Unquote keyword arg names
Error if a user defined function uses a reserved name
Should error when trying to string concatenate with null
Merge pull request #727 from xzyfer/feat/reserved-function-names

Error if a user defined function uses a reserved name
Merge pull request #728 from xzyfer/fix/concatentating-null

Error when trying to string concatenate with null
Merge pull request #710 from mgreter/feature/default-args-cust-fn

Fix default argument passing for custom c functions
Merge pull request #716 from mgreter/fix/fractional-truncated-zeros

Fix fractional output for truncated zeros
Merge pull request #691 from mgreter/feature/importer-parent-context

Improve custom importer to pass parent import context
Merge pull request #715 from mgreter/feature/overload-error-fn

Implement @error function
Add const correctness for sass_values

This should help to avoid issues like #689
Compiler should emit a warning in that case
Merge pull request #692 from mgreter/fix/const_sass_values

Make Sass_Values const in API function calls