Extension namespace fix to be PSR-4 compatible after monorepo merge

[ek9]

This is a big PR with the following changes:

• The namespaces have been updated to be compatible with PSR-4 and reflect the updated structure after everything has been moved to monorepo. This mostly means that everything is under RMiller\BehatSpec now (src/), thus src/Extension becomes RMiller\BehatSpec\Extension. This was a bigger change, but I have updated all the related composer.json files + tests + retested everything to make sure all of the extensions work fine.
• PHPSpec related extension requirements updated PHP requirement from 5.4+ to 5.6+ (phpspec3 requires 5.6+)
• Update composer.json of behat-spec package to efficiently and correctly load extension via PSR-4
• Updated README.md to ask using v0.3.* for PHPSpec2 support until v0.4.* is stable.
• Updated README.md in main and all extensions to use the new namespaces

This is related to and fixes #18 #23

I have made sure all the tests pass in all extensions where the tests were setup. There is still ExemplifyExtension tests failing, but it's same failures that were there before this PR (I've made a sparate issue to track it)

[ek9]

I belive src/Extension directory is setup as part of subtreesplit. I would suggest to get rid of that instead and use src/<EXTENSION> structure. We could then also remove the Extension sub-namespace to clean it up, however that would require further updates to subtreesplit + break compatibility with 0.4

[ek9]

This would fix #25

[ek9]

This would also fix #5 as I have tried running the updated version on an existing behat/phpspec heavy project which previously relied on phpspec2 and it seems to work fine with PHPSpec3 so far.

[richardmiller-zz]

Brilliant, thank you for this!