Added url() value type with class UrlValue. Urls can be used in
functions imgwidth() and imgheight() to get the width and height of an
image resource.
Added support for jpeg images.
Moved to autotools build system to make building easier.

- Added configure flags --without-libpng and --without-libjpeg to
  build without those libraries.
Moved unit tests outside the src directory and changed the build
process to make the tests work.

The main source files(without main.cpp) are compiled as a library now
so that the test can link to it.
Added header files to sources in the Makefiles.
Renamed main.cpp to lessc.cpp.
Added files generated by autotools to .gitignore.
Moved example files to example directory.
Merge branch 'extra_features' with master.

- Adds imgwidth() and imgheigh() functions to less syntax.
- Adds autotools build process.