Add tests for repr on tables

When a table is backed by mutable data, repr(table) raises a TypeError
about hashing an unhashable type
Add into for DataFrame to DataFrame
Fix errors caused by hashing data in head

The data backing a table was being hashed while computing head. This
commit changes the traversing behavior to avoid this, and also changes
the hashing behavior for Table.

The hash of a Table object is now a hash of the schema, the name and the
id of the data.
Minor cleanup
Fix python3 incompatibility in table test
Added unicde to requirements and docs
Added unicodecsv to conda.yaml
fix path when running nosetests outside of the test directory
Merge pull request #383 from cpcloud/fix-csv-path

Use the correct path when running nosetests outside of the test's directory
remove old source files
use relative paths for header links

Otherwise the dev and core docs link back to core
update requirements
update developer workflow
Merge pull request #379 from davclark/master

Added unicde to requirements and docs closes #378
into: DataFrame <- Series
expr: break out subs to dispatched method
api: Redefine Table.args to avoid possibly mutable data

This required redefining subs.  Everything else seems to work well
expr: scalar date/datetime coercion
expr: basic unsafe exprify implementation
server: Selection API