Questions containing fractions should return fraction result

# Goodie Pull Request Template

Using this template will help us better understand your Instant Answer and assist you when necessary.  Simply copy and paste the markdown below into the description of your GitHub pull request and complete as appropriate.

**What does your Instant Answer do?**
Returns a fraction math result if the query contains fractions

**What problem does your Instant Answer solve (Why is it better than organic links)?**
Currently the calculator only returns decimal answers, this feature adds the answer in fraction form

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N/A

**Why did you choose this data source?**
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**What are some example queries that trigger this Instant Answer?**
"3/7 + 5/6", "1/4 + 2/4";

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Anyone who has the need for a calculator

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Yes

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## Checklist
Please place an 'X' where appropriate.

```
[] Added metadata and attribution information (https://duck.co/duckduckhack/metadata)
[X] Wrote test file and added to t/ directory (https://duck.co/duckduckhack/goodie_tests)
    Appended tests to the existing Calculator.t
[X] Verified that instant answer adheres to design guidelines (https://duck.co/duckduckhack/design_styleguide)
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[X] Tested cross-browser compatibility

    Please let us know which browsers/devices you've tested on:
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        [] Google Chrome
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        [X] IE 10

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```

For irrational numbers, an approximation is given.  I just committed the change so that only a max of 15 terms will be generated in the compute_continued_fraction_terms sub for any given number.  So for 31425926353/10000000000 the result is 506021593476/161020422371