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
```

hmm, weird that it passes for me. I've 0xBE for all 50 bytes. Maybe something related to alignment. I'm on Ruby 2.1 x64 with Libav also compiled for x64

I'll update this test