ARROW-2153/ARROW-2160: [C++/Python] Fix decimal precision inference

[cpcloud]

No description provided.

[cpcloud]

This requires ARROW-2145 to address.

[wesm]

Do you want to wait for ARROW-2145 or merge this independently?

[cpcloud]

I have a couple more tests to add here

[cpcloud]

@wesm @pitrou I reintroduced boost regex for parsing the decimal number, to address ARROW-2153. We were using a fairly straightforward algorithm before that implemented the regular expression by hand but it didn't allow numbers like 1e1. The regular expression to match that would be very complex and hard to read if written by hand so I decided to use boost regex. I didn't go with STL library because it doesn't support named capture groups, which I think make the code much more readable.

[wesm]

std::regex is totally broken in gcc 4.8.x (what we're using for conda/pip releases) AFAIK so using <regex> isn't even an option right now. When we get past gcc 4.8 it might be nice to use the STL regexen

[cpcloud]

Yep, I believe std::regex_match is implemented as return false; :(

[cpcloud]

In gcc 4.8 that is. 4.9 may be implemented, but I don't think it supports named capture groups.

[wesm]

needs rebase

[cpcloud]

Closed in favor of #1651