Make addon more idiomatic at the expense of making it 2.10+ & fastboot 1.0rc+

[cibernox]

Added a note on the readme explaining that.

[kratiahuja]

Might as well move app/instance-initializer/fastboot/head.js to fastboot/instance-initializer/head.js since this is a major version bump ?

[cibernox]

@kratiahuja I don't follow. That is precisely what I did.

[kratiahuja]

Super sorry :( those changes showed up very small in the diff so I got confused. This LGTM.

[simonihmig]

@cibernox can you please explain the rationale for these changes? A bit of work went into #21 to not make any breaking changes, so it now runs with beta and rc version of FastBoot as well as those older Ember versions.

I understand the required code became quite a bit confusing when looking at it, but as this addon is probably used by most FastBoot apps, I think a higher degree of compatibility would make sense. At least until we feel safe to drop support for older version, say when FastBoot 1.0 stable has been released and most other addons have been migrated (still a long way, see ember-fastboot/ember-cli-fastboot#387). Then the changes here make absolutely sense to me!

[cibernox]

I just asked @ronco if leaving 0.2.2 as a bridge version and release a simpler and leaner 0.3 version that is 1.0+ only was ok. He seemed to be ok, people using fastboot beta can or non-glimmer ember can continue to use the old version.

This PR also makes #30 irrelevant, so it's like 3 PRs in one.

[ronco]

LGTM. @simonihmig are you ok with dropping pre-1.0 Fastboot support with a 0.3 version bump?

[simonihmig]

@ronco Sure, if it's ok for you, so it is for me! :)

[rwjblue]

agreed, I like this direction