Pycrunchbase

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* Add `series` to the `FundingRound` node.

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* Update `__str__` for nodes and relationships

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* `Relationship` is now a subclass of `Page`, although this strictly isn't true.
The benefit is that this allows us to reuse a lot of logic.
Relationship can be thought of as Page 0, which is a summary of potentially
multiple pages of `PageItem`. The only time we get a relationship is when we
query for a particular `Node`, e.g. organiation, and we grab the relationships
returned by the API. After this, to get more details we call `Crunchbase.more`,
and this returns us a `Page`.

* Added `__repr__` methods to all the `Node`, `Relationship`, `PageItem`.
Previously we only defined `__str__`, but these didn't show up in places
like the REPL. This fixes that. We try to make it obvious what object it is
based on what is printed, but also don't want to be too verbose.

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* `InvestorInvestmentPageItem` now has the possibility of being either a
`investor`, or a `invested_in` relationship

* Propogates any exception when making the actual HTTP call to CrunchBase

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* Add a `cb_url` attribute for all PageItem, this url is a CrunchBase page
(not the API) that holds more information for a particular PageItem
Allows you to make calls like::

company.funding_rounds[0].cb_url

to get the url of the page for the first funding round of `company`.

* A new page item, InvestorInvestmentPageItem, that is useful for FundingRound info::

round = cb.funding_round('round_uuid')
an_investor = round.investments[0] a InvestorInvestmentPageItem
print(str(an_investor)) prints: Investor Name $100000

* Add simplified Contribution guidelines in README

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* Relationship retrieval is 0-based now, 1-based just doesn't fit well with array
* Better `__str__` for `Node` and `Relationship`
* `Relationship.get(i)` if `i` is too large or small will return a NonePageItem singleton