Move constants out of global namespace

[MatheusRich]

Defining constants in the global namespace is dangerous as it might conflict with other libraries/apps.

Note: This is a breaking change.

[MatheusRich]

This could be a module instead, since we're not creating any instances

[vkononov]

Given that this is a break change, did you actually encounter an issue where you had a conflict with another gem or your own code?

[MatheusRich]

Yes, and I think this will keep people from using this gem.

[vkononov]

Thanks for the pull request. I will try to review and merge it right away and release a new version of the gem.

[vkononov]

The changes look great. The only thing that is missing is updates to README.md. Would you be able to make those changes?

[vkononov]

I was trying to resolve conflicts between this branch and the previous branch you had pushed, which I recently merged, and it looks like it broke something. :(

[MatheusRich]

@vkononov I think the tests failing are because we've merged #37. I'll rebase.

[MatheusRich]

@vkononov I've updated the references to Field to FillablePDF::Field in the README. Is there anything else you'd like me to change?

[vkononov]

Looks great. Thank you, @MatheusRich, for making these changes. Will merge right away and release a new version of the gem sometime today.