Fix irregular Timestamp arithmetic types #6543
closes #6543

This is my first PR so I may have done a few things wrong. If I should have added my tests to a different file/location, please let me know.

I should point out that this code is also tested by many other sections of the pandas codebase, on my first pass I hadn't captured the "Timestamp - datetime" case and all sorts of things blew up. Better now. :)

no exactly sure why travis doesn't run your branch (its supposed to)...but this fails under numpy < 1.7

https://travis-ci.org/jreback/pandas/jobs/20093901

need to put a conditional not to test under 1.7 (search for `_np_version_under1p7`) and skip that portion of the test (its the broken timedelta support that causes an issue)
 @jreback this reminds me: I was looking at `BlockManager` implementation the other day and it seemed to me that it was too busy micromanaging type coercions & casts. I though that pandas could benefit by wrapping all the nasty `numpy` interaction details in a single module that would provide a `pdarray` subclass of `np.ndarray` with all the necessary casts and workarounds already applied (i.e. boxing/unboxing values, proper datetime parsing, type inference).  It could also provide basic building blocks for more advanced algorithms, e.g. pandas-dtype-aware concatenation. 

This seems an obvious application of single responsibility principle, did anyone consider/start doing this before? Were there any pitfalls that prevented that?
 I stand corrected: single responsibility principle is more about OOP, I meant [separation of concerns](http://en.wikipedia.org/wiki/Separation_of_concerns).
 @immerrr and how would this be different than the `Block` types? that's really their main purpose.

more interesting would be to cythonize `BlockManager` (but a bit of a project!)
 This is not really about `Block`s, but rather about handling `np.datetime64`, `np.timedelta64` and `np.bool_` -related workarounds in a centralized manner. For a person outside of project's scope all that magic is impossible to follow, because they're distributed evenly all over the code base.

This would require proxying all objects and function imports that may relate to numpy via some `pdnumpy` package. This is quite a project, like you say, but it would demarcate an area inside `pandas` and within that area everything would behave exactly as expected. This would allow to abstract away those gory details and focus more on the actual value added by `pandas`.  Do you agree?
 @immerrr makes sense. Though one could argue that these are these are actually conventions that pandas uses to deal with numpy. Not bugs per se, but more 'workarounds' to treat things in a nicer manner (e.g. taking views on datetime64[ns] and such). IMHO this would only be useful as a layer above numpy, but I am not sure of the utilitiy for non-pandas (my 2c).

Centralizing this would be a very nice feature as they _are_ pretty scattered. I am all for elegant code, so I would be +1 on this. 

Their was some code in `core/array.py` to proxy out `ndarray` in general (should prob be removed as its only used in a very way and doesn't really do anything, side -issue). The series not a sub of ndarray mostly 'fixed' the need for this (as pandas I don't think can really ever divorce from numpy).
 I am new to Travis, hopefully I got it right this time - looks like the same build works after my latest change?

https://travis-ci.org/rosnfeld/pandas/jobs/20135641
 yep looks good

pls rebase and squash to a single commit, see here: https://github.com/pydata/pandas/wiki

(I can do it for you when you merge, but its nice to try to do it yourself, makes a clean workflow)

essentially

```
git rebase -i origin/master
```

squash

```
git push myfork mybranch -f
```
  Ok, I hope I got it right. The command you posted has me rebasing on my own fork, but not incorporating the latest upstream commits, right? So you may need to do a tiny bit of work to merge release.rst ..?
 you can pull in the master changes by doing

`git pull origin/master`

or do

`git pull origin/master --rebase`
 I think you mean "upstream/master" for me to incorporate new items from others? "origin/master" is my fork and doesn't have anything new.

Sadly while the change (using upstream/master) looks good to my eyes, Travis seems to be upset. I don't understand why.
 yes..`upstream/master`....rebase and force push again...
 Ok, I am confused now. Not sure why a second rebase did anything, though it did, and now this PR shows everyone's commits instead of just my own (probably because my branch is now "ahead" of my origin/master). I guess I would have expected this result after my first rebase. Meanwhile, Travis is still upset.
 merged via https://github.com/pydata/pandas/commit/baa4c1d0da21df1ac14712f9fe428ac8f6e025fa

@rosnfeld git takes some getting used to......but great job on this PR!

thanks!
