Travis rbx fixes
Add ruby 2.1.0 to tests
Merge pull request #271 from jurriaan/travis-fixes

Travis fixes
Update header_footer.rb

Fix documentation.
Fixing typo in documentation. ws.styles!!

Replaced ws.style with ws.styles.
Merge pull request #275 from sudhirrs/patch-1

Fixing typo in documentation. ws.styles
fixing comments for 2010 starter edition
Merge pull request #279 from dustinsmith1024/comments-fix

fixing comment issues, see #170
[#280] Updates rubyzip dependency
Column_widths must be set after add_row is called

Fixes https://github.com/randym/axlsx/issues/165
Merge pull request #282 from zyphlar/master

Column_widths must be set after add_row is called
Merge pull request #281 from pirvudoru/axlsx-280

[#280] Updates rubyzip dependency
Do not create enormous strings during serialization
Fix string concatenation
Fix sanitizing
String concat optimization
Freezing and adding some constants
Let SimpleTypedList inherit from Array

And some other optimisations
Reduce RAM usage

By not initialising instance variables when they are not needed
Huge refactoring