Detect uppercase date time formats

[zeehio]

Hi,

The dates-1900.xls once opened and saved using LibreOffice 5 was not readable by readxl (it detected dates as numbers).

This patch allows to detect uppercase date time formats (used at least by libreoffice). A test case is included. Note that the test fails due to different autogenerated col_names in xls and xlsx

EDIT: Excel is able to parse the file so I assumed that readxl should be capable too, otherwise the pull request would have gone to libreoffice.

[MichaelChirico]

Note that this closes #134.

@zeehio I guess it's best to amend the commit message so the issue closing happens automatically?

[zeehio]

I updated the commit message. Please note that readxl has been like this for 8 months, so the chances of this being merged in the next few days are not huge. Hopefully eventually someone will work a bit on the package and will deal with the increasing number of pull requests.

(I am just another random readxl user, there is not much more I can do)

[MichaelChirico]

I hear you, there are a couple of good PRs that would improve the package a lot but it appears it's been on the shelf for some time

[jennybc]

@MichaelChirico readxl and these PRs will get handled in early 2017

[hadley]

Looks good. Can you please add a bullet to NEWS.md?

[zeehio]

Rebased on master and done