BUG/TST: Allow generators in DataFrame.from_records
closes #4910
- nrows implementation doesn't allow unknown size iterator like
  generators, if nrows = none ends with a TypeError.
- To allow generators if nrows=None consume it into a list.
- Add two tests of generators input.

pls add release notes (`doc/source/release.rst`), referencing the issue
pls hook up travis (see contributing.md)

thanks!
 Sorry for the mess, I'm new to github. 
I've tested it locally with python 2.7 and 3.3 but didn't work with 2.6. Now it's all ok.
 I recognize that this may be silly, but how is this supposed to work if you passed something like this:

``` python
gen = ('a' for _ in range(50))
DataFrame.from_records(gen)
```

Just wondering if this needs to be caught anywhere or if this is okay
 I suppouse it's ok, the patch only put the generator in a list in the same fashion as main constructor.
DataFrame.from_records() expects a collection of records, which every record it's a collection of fields: list, tuple, dict, or in your case a string wich it's a sequence (collection) of characters (only one character in your example).
 it's fine, it looks like it just works the same as a list. Doesn't need to be added as a test case either.
 @tinproject can you rebase and squash to commit? otherwise looks ok
 Hope it's ok, if it's not please give me a git command example, as i said i'm new to git :(
 pretty good instructions here: https://github.com/pydata/pandas/wiki/Using-Git, let us know 
 I think it's now ok. Release notes file release.rst conflicts but are trivial and I thought I solved it.
 try `git rebase -i upstream/master` need to remove that merge of master commit
 @jreback now it's all ok, no? I must improve on git, sorry.
 yes...looks good.....let me review
 git just comes from practice! submit more PR's!
 @tinproject thank you!
