The go command on Windows doesn't like long path name. To
work around this, this is the first in a series of changes to
shorten the schema disk path structure in a few places.
Fixes#89
This adds support for extracting a cell's formatted value according to
the number format applied to the cell. To do this we need to implement
a parser for Excel style format strings and support formatting numbers
according to that style.
This also enhances the General formatting to be much closer to what
Excel normally does. There are likely still a few corner cases where
Excel and gooxml differ, but hopefully not too many.
