Improves error handling in c sass_interface.cpp

Handle all exceptions and do not let them propagate to
any underlying C code, as it will terminate the program.
Changed some messages and made all starting with uppercase.
Changed "source string" indicator to "stdin".
Allows C bindings to overwrite input_path on sass_compile

Fixes a bug when entry_point is implemented by C bindings
for string compilations (libsass tries to read that file).
Fixes some clang warnings
adding a note about the implementations wiki page. closes libsass#125
Merge pull request #496 from mgreter/feature/better-error-handling

Improves error handling for C Binding
Adds gcov coverage to autotools build
Enables coveralls.io in travis ci config
Merge pull request #497 from mgreter/feature/test-coverage

Enabling coveralls.io
Allow trailing commas in maps
Merge pull request #476 from am11/master

Interface: Moves omit-url to options struct
Merge branch 'xzyfer-featt push origin master/maps-trailing-comma'
Merge branch 'master' of github.com:hcatlin/libsass
we dont follow those install rules
Update bootstrap

Forgot to update this with the last release.
Fix CreateFileW function usage, otherwise it fails to open file.
Removing emscripten stuff. Closing #419
Merge branch 'master' of github.com:hcatlin/libsass
adding a note about null values in the options objects as per #204
added more documentation to the sass_options as per #202
Merge pull request #501 from vadimi/master

Fix CreateFileW function usage, otherwise it fails to open file.