Adding pkg/marc unit testing

[blackerby]

This pull request adds unit testing to pkg/marc. It adds five files and one directory to the pkg/marc root directory. Those files and directory are:

• field_test.go
• filters_test.go
• leader_test.go
• marcfile_test.go
• record_test.go
• testdata/, which has copies of the files in the marcli/data directory and adds:
  • bad, an XML file without an XML extension
  • test_bad.xml, a MARC binary file with an XML extension

The PR also includes an addition to the .gitignore file, adds https://github.com/google/go-cmp as a dependency for testing, fixes some compiler warnings and typos in the comments, and lightly refactors some of the library files.

Some behavior remains untested, primarily the error handling code.

Closes #12

[hectorcorrea]

I probably won't look closely into this until after Code4Lib, but it looks good. Thanks for submitting it.

[hectorcorrea]

@blackerby sorry for the long delay. Thank you so much for the work on this PR.

[blackerby]

Happy to contribute!