Refine polymer project templates

@devoncarew @keertip (whichever of you wants to take a look)

 Much improved handling of problematic Bower dependencies

1) Added a capability to override or ignore problematic dependencies via a set of new config flags.
2) Added new handling logic to cope with unsupported complex package versions and `"*"` paths.
3) Added an extensive post-download diagnostic report telling what dependencies have been altered, ignored or remained unresolved, with instructions on how to manually fix or override that.
4) Added `bower_config.json`, which overrides certain third-party dependencies imported by Polymer templates (those have `"*"` as their path, which means "ask Bower registry", which we can't yet do).
5) Use corrected sets of dependencies for Polymer project templates.