Behapi v0.12.0

This release is mainly a work on contexts. It may contain a BC break or two, as some contexts were "removed", "renamed" or "changed".

This release also uses beberlei's asserts instead of webmozart, because of its builders abilities for assert (Through Assert::that()), it has more assertions than the other one, and it seems to be more maintained too (even though @Nyholm's taken the project now).

Changes (BC Breaks included) since v0.11:

• Split some assertions groups into their own traits. They'll probably be on their own context in the future (#52)
• Use beberlei/assert instead of webmozart/assert (#53 #38)
• Fix the assertion on maxCount (0b999ee)
• Merged most collection / root steps (#52)
• Merged most positive / negative assertions (#52)

Removal

• BC Break (soft): The Behapi\Json\AbstractContext context is
  gone and has been merged into Behapi\Json\Context (#52)