Overhaul the gem directory structure, add Travis and Gitlab build support

[cheald]

This PR updates the gem structure to be more idiomatic:

• The C extension is under ext/
• The Ruby portions of the gem are under lib/
• Added test_helper for test setup, replacing FbTestCases

This also updates the tests a bit to work better in container isolation:

• DB paths are now relative to the build directory
• The rubytest user is dropped if it exists, then recreated as needed. This allows testing in both aged and pristine environments.
• The test suite no longer uses the source files as test data for tests.

Finally, this adds support for building on Travis and Gitlab CI on Ruby 1.9, 2.0, 2.1, 2.2, 2.3, and 2.4.

It currently builds against Firebird-2.5, but future improvements could include CI for FB-3.0.

[mariuz]

I have added them to my fork , I will add there Firebird 3 testing in travis/github