Prawn::Icon::Base [#27]

Bump version to v1.2.0.
Update FontAwesome from v4.5.0 to v4.6.3.
Refactor spec files to remove duplication using let blocks.
Break internal dependencies to Prawn::Icon::Base to allow selective code requires. (see issue #27).
Introduce Prawn::Icon::Errors to contain internal exception classes.
Prawn::Icon::FONTDIR is now an alias to Prawn::Icon::Base::FONTDIR for compatibilty.
Add basic spec files for Prawn::Icon::Base and separate exception classes.
Minor backwards-compatible code refactor of Prawn::Icon::FontData for readability.
Point legend rake task code to Prawn::Icon::Base::FONTDIR.
Add a spec to test inline_icon with final_gap: false to achieve 100% coverage.

jessedoyle / prawn-icon