Load libraries with :require instead of :use.
Make headers type string by default instead of keywords.

Standardizes read-dataset for a more uniform api.
If no header function is supplied, headers are string by default.

Before:
  If :header option was false, by default the headers were strings.
  If :header option was true and there was no :header-fn option
          default :header-fn was keyword.

Now::
  If :header option is false, by default the headers are strings.
  If :header option is true and there is no :header-fn option
          default :header-fn is identity (in essence string).
Change get-dataset to use new read-dataset API.

Adds tests for incanter.datasets namespace. Includes test for all
Incanter provided datasets.
Only the iran-election dataset is not tested due to unmatched column
counts. This dataset is unreadable by the CSV parser and must be fixed.

The incanter.datasets tests use the local version of Incanter datasets
instead of retrieving the datasets from the repo. This makes running the
tests faster and facilitates possible format changes to the datasets and
testing the changes.

Formatting of many datasets was fixed so they could be read by
clojure.data.csv/read-csv. Mainly extra spaces were removed in files
with \space (single space) as the delimiter.
Fix errors in charts unit tests.

Tests now read from the local datasets instead of retrieving them from
the repo.

Columns that are numbers are explicitly converted to double.
The bar-chart and line-chart functions expect values as numbers (more
specifically the underlying jfree DefaultCategoryDataset being
constructed), this caused errors since all fields were strings.

The exact issue was that the `addValue` method of DefaultCategoryDataset(
http://www.jfree.org/jfreechart/api/javadoc/org/jfree/data/category/DefaultCategoryDataset.html#addValue-double-java.lang.Comparable-java.lang.Comparable-)
expects either a Number or double as the value, therefore we have to
convert the fields accordingly.

The ideal way would be to pass in a :row-fn when reading the dataset to
convert columns to their corresponding types.