[Excel 2003 XML] Add support for font colors + required fixes

[ms-ati]

As mentioned in #42, all I'd needed was to add support for the color of a given cell. But unfortunately, I discovered that other pieces of the puzzle were broken along the way.

This PR attempts to fix all that, adding the support for fonts and colors via #font.

Adds RSpec coverage via the following test files (screenshot included for those without Excel):
font_colors.xml.zip

Fixes #41, #42, #43, #44

Also fixes #33 I believe

[ms-ati]

Following semantic versioning, I recommend bumping a MINOR version, to 1.2.0 after this PR is merged, as this code adds new capability (font name, size, and color) into an existing API, and fixes existing bugs.

However -- there may be users depending on the mis-parsing of the cells, especially row and column numbers around merged cells. Therefore, you may need to bump a MAJOR version, to 2.0.0, to reflect incompatible changes for some users.

Does that make sense? Depends how you all are thinking about it.

[coveralls]

Coverage increased (+1.2%) to 80.977% when pulling 5a06c76 on ms-ati:excel-2003-xml-support-font-colors into 00ccb4c on roo-rb:master.

[ms-ati]

@Empact et all -- any chance of getting eyes on this PR?

[Empact]

This looks great. Thanks! Keep the PRs coming. :)

[ms-ati]

Hurray! Thanks @Empact. I might try to tackle the streaming api question, now that you mention it. In the meantime -- our team would really appreciate it if you could cut a gem release that contains this PR?

Cheers!

-Marc

[ms-ati]

Ping on request for a roo-xls gem release @Empact ?

[Empact]

@ms-ati Released! Could you give it a try? https://rubygems.org/gems/roo-xls/versions/1.2.0

Also a github release /~https://github.com/roo-rb/roo-xls/releases/tag/v1.2.0

[ms-ati]

Nice @Empact ! Thank you :)