Add minimum support for Bitrise.io

[mstachniuk]

In scope

The minimum support for Bitrise can be implemented similar like TravisPlugin class. See it for details.
In Bitrise the environment variables are different: https://devcenter.bitrise.io/builds/available-environment-variables/

Mapping:

Travis	Bitrise
TRAVIS_BRANCH	BITRISE_GIT_BRANCH or BITRISEIO_GIT_BRANCH_DEST
TRAVIS_PULL_REQUEST	PR
TRAVIS_COMMIT_MESSAGE	BITRISE_GIT_COMMIT

Of course, it needs to be tested on Bitrise.

Outside the scope

The Bitrise users usually don't store bitrise.yml in the same repository as code like in repo build by Travis-CI. They use GUI for defining build steps.
More details: https://devcenter.bitrise.io/tips-and-tricks/use-bitrise-yml-from-repository/
Because of that generation of bitrise.yml is out of the scope of this issue.

Workaround for now

If you like to use Shiplkit on Bitrise please define in step before the build environment variables used by TravisPlugin. See the above table with mapping.
Any feedback is welcome, please comment on this issue.

[mstachniuk]

Example project, how to use Shipkit with Bitrise.io: https://github.com/mstachniuk/shipkit-bitrise-example