Release 0.9.4

* Formula support. Formulas used to cause things to blow up, now they don't!
* Support number types styled as dates. Previously, the type was honored
  above the style, which is incorrect for dates; date-numbers now parse as
  dates.
* Error-free parsing of empty sheets
* Fix custom styles w/ numFmtId == 164. Custom style types are delineated
  starting *at* numFmtId 164, not greater than 164.