Make empty cells with specific types not raise

Previously, if a cell had a type but no value, there would be an xcell
with no 'v' element, causing the spreadsheet to not parse.

Fixes github issue #3
Add a performance test

Tests that spreadsheet parsing happens in linear time relative to the
number of rows (number of columns are constant in the test).

This is the start of trying to improve speed. Made one attempt that
broke linear parsing, plus you can use the raw numbers in the output
to eyeball improvements.
Release 0.9.7

Fix parsing empty cells with specific formats (was blowing up the
parser)
Rubyzip 1.0 compatability

For now, we also maintain compatability with versions < 1.0 so we don't
force anyone into a dependency hell for no real reason, since the
rubyzip API didn't change except how they name their core classes.
Release 0.9.8

* Rubyzip compatability
Upgrade to minitest 5.0

Since it wasn't specified in the gemfile, devs trying to use 5.0 would
get a big fail when checking out the project & running `rake`.

This fixes both the benchmark test (which needed to have the word
'benchmark' in the test name :/) and the warning for "using
minitest/autorun instead" (which we were, but in the wrong order
apparently)
Fix merge conflicts