Fix Tests

`rspec its` is used, but it wasn't added as dependency, so added it. Then in `spec_helper` it's set to use _expect_ syntax, but that file wasn't actually included in tests and there was still _should_ syntax used. So I added to use `spec_helper` and rewrite _should_ to _expect_ syntax.

If Ruby's default encoding is other than `US-ASCII/ASCII-8BIT`, then `match(/\xBE+$/)` will fail with `invalid multibyte escape: /\xBE+$/ (SyntaxError)`. Also it would match even if there would be just one 0xBE. So I rewrote that as `0xBE.chr * 50`

In some places `be` was used while actually should use `eq`, difference is that `be` matches exact instance while `eq` only value, so `expect("a").to be "a"` would be false because it's different object.

With this PR, all tests does pass `197 examples, 0 failures`.

Also added default rake task which lists all tasks so executing just `rake` will show
```
rake build    # Build ffi-libav-0.3.0.gem into the pkg directory
rake install  # Build and install ffi-libav-0.3.0.gem into system gems
rake release  # Create tag v0.3.0 and build and push ffi-libav-0.3.0.gem to...
rake test     # Run tests
```

This test change fails for me.

```ruby
  1) Libav::Frame#scale scale(:scale_ctx => ctx) contains a 100x100 image
     Failure/Error: expect(subject.data[0].get_bytes(100, 50)).to eq(0xBE.chr * 50)

       expected: "\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE"
            got: "\x00\x00\x00\x00\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE\xBE"

       (compared using ==)
     # ./spec/frame_spec.rb:270:in `block (3 levels) in <top (required)>'
```

The reason I was using `match` instead of `eql` is that I had experienced the leading four bytes being set to zero.  I'm assuming this is an artifact of the scaling.

You can probably change this test to an `include()`, and reduce the number of bytes that much match from 50 to 45.