use lesscss for building CSS, add less files from components we use
less filter: enable source map when assets are in debug mode
assets: remove all _CSS definitions. All css files renamed with .less extension
style: use less syntax, added variables.less to overrides definitions from included packages
fix CSS causing trouble when parsed as a less file: removed rules for IE5.5 support in datatable theme
assets: preserver order of css import; fix wrong sources references in css source map file.

Now all CSS are imported as LESS content, so that url() values are properly rewritten; as a bonus order of inclusion is preserved (not the case when importing as CSS content.
LessImportFilter: use LESS_RUN_IN_DEBUG option
assets: removed all JS_DEBUG entries. When production mode is on closure compiler will minify all
add install intructions for lesscss dependency
assets: less filter is now used on output; fix 'less_import' filter to pass-through when not used on real file
assets: SubBundle class is not needed anymore; all LESS assets are simple Bundle instance (again)
app.register_asset: `assets` parameter is now *args
update bootstrap-switch to 3.0.0
fix doc markup
doc: add intersphinx ref for `webassets`
update CHANGES: css style switched to lesscss
Merge branch 'feature_use_lessc'