fix(inline): Inline Icon Page Wrapping

Previously inline formatted icons would not correctly start a new page when necessary, so the formatted text box would render at a cursor position that made it invisible.
Add a new test case for the bug outlined in #49.
Adjust #inline_icon to directly call Prawn's #formatted_text method with the transformed inline formatted string.
The previous implementation of #inline_icon went to great lengths to position the formatted text box without rendering to the document, but Prawn's formatted text box wrapping logic may require multiple Prawn::Text::Formatted::Box instances (around page breaks).
Due to this change #inline_icon calls now return nil and content is rendered immediately to the document.
This change also means that #icon calls will return nil when using the inline_format: true parameter.
Add a #formatted_icon_box method that retains the previous inline icon behaviour.
Allow #formatted_icon_box to accept the :at, :x and :y absolute positioning parameters (Thanks @navisnspm!).
Transition to Pdf::Inspector to make positioning decisions in specs. This is a more comprehensive test expectation.
Change the deprecation message for the FontAwesome 4 shim to include a future 4.X version as the compatibility layer will be maintained for at least another major version.
Update major version to 3.0.0 as the return values have changed in the public API #icon and #inline_icon methods.

jessedoyle / prawn-icon