Fix for empty code fences

When there's an empty code fence, polytexnic breaks:

```
/usr/local/var/rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/polytexnic-0.9.3/lib/polytexnic/utils.rb:209:in `add_font_info': undefined method `gsub!' for nil:NilClass (NoMethodError)
	from /usr/local/var/rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/polytexnic-0.9.3/lib/polytexnic/utils.rb:141:in `block (2 levels) in highlight_source_code'
...
```

To replicate, I'm just doing something like:

```ruby
```
Include a file from a git tag

This change allows inclusion of code from a specific git tagged version
of a file.  It looks for the given tag/filename in the local git repo;
it is not currently possible to specify a different repo.

The file is checked out into a temporary directory and included into the
book, and then the temp directory is deleted.  The operation thus has
no effect on the state of the repository.
Use FakeGit in tests
Merge pull request #102 from benjamintanweihao/patch-1

Fix for empty code fences
Extract CodeInclusion module into separate file
Add a failing test for broken table

[#70476292]
Fix tables with center env

Work by reducing to a previously solved problem (for figures).

[Delivers #70476292]
[#70476292]

* 70476292-table-xrefs:
  Fix tables with center env
  Add a failing test for broken table
Add a kludge for footnotesize in tables

[Delivers #70476292]
[#70476292]

* 70476292-table-xrefs:
  Add a kludge for footnotesize in tables
Fix encoding
Bump version number
Move footnote nodes out of chapter anchors

[Delivers #70812746]
[#70812746]

* 70812746-footnote-chapter-headings:
  Move footnote nodes out of chapter anchors
Rescue nil when missing chapter number

[#70812746]
[Finishes #70812746]

* 70812746-footnote-chapter-headings:
  Rescue nil when missing chapter number
Merge pull request #103 from skmetz/include_file_by_git_tag

Include a file from a git tag