Add a subtotal option to PivotTable (dirty).
Allow overriding the 'created' timestamp in the docprops.

Can be specified as option to Package#new:
```
Axlsx::Package.new :created_at => time
```

If omitted, the current time at the moment the document is serialized will be used. This change is therefore fully backward compatible.
Merge pull request #195 from die-antwort/allow-override-for-created-at

Allow overriding the 'created' timestamp in the docprops.
Modify pivot table caption according to subtotal function.
Allow a different subtotal function for each data field.
Fixes Issue #202 Axes are borked in Bar3DChart
provide a better default for dispBlanksAs and allow it to be configured
fixed bad test case and bad code for dispBlanksAs (setting now actually makes it into XML)

This means there are many other test cases in the suite that are not verifying what they look like they're verifying.
added sparse array transposition with blocks for rows/cols switching and some docs updates for release prep
Merge branch 'master' of https://github.com/randym/axlsx
fix typo in specs
escape formula for conditional formatting
provide a better default for dispBlanksAs and allow it to be configured
Merge remote-tracking branch 'randym/master'

* randym/master:
  escape formula for conditional formatting
  fix typo in specs
  added sparse array transposition with blocks for rows/cols switching and some docs updates for release prep
Fixes Issue #202 Axes are borked in Bar3DChart by requiring axis order in the constructor
Fixes Issue #202 Axes are borked in Bar3DChart by requiring axis order in the constructor
Merge pull request #203 from moses/master

Fixes #202 Axes are borked in Bar3DChart
Merge pull request #209 from moses/fix-axes-ordering

Fixes Issue #202 Axes are borked in Bar3DChart
Merge pull request #205 from moses/configurable-display-blanks-as

provide a better default for dispBlanksAs and allow it to be configured
Merge pull request #194 from sdeframond/subtotal

Add a subtotal option to PivotTable.