Outline new API based galaxy interactor for functional tests.

Add mechanism to tool test parser code to determine which Galaxy interactor should be used ('twill' or 'api' are current options). This maybe a stop gap to force usage of the API interactor until that becomes the default or may prove in the long term to be essential if there are certain tools that will always require Twill-specific functionality or if new browser-based JavaScript (e.g. w/selenium) are implemented. An interactor value can be specified at the tests level and/or at the level of individual test elements in the tool XML (using the 'interactor' attribute on either element).

The current default interactor is 'twill'. The default interactor app-wide can be overridden with the GALAXY_TEST_DEFAULT_INTERACTOR environment variable.
Twill-less tool tests initial implementation.

Implement GalaxyInteractorApi, is working for simple tests.

Lots left to do likely - conditionals, repeats, composite uploads, pages(?).

Introduces dependency on Python requests package (needed-ish for API uploads).
Move test_toolbox's expand_grouping out into tool/test.py.

Would be good to merge these two implementations there at very least easier to reuse/test in its new home.
Simplify logic in tool test expand_grouping.
Twill-less tool tests - handle repeats and conditionals.

Expand 'inputs' tree when building tool inputs for API functional tests.

Code is in there for both flat inputs (the way the UI would submit them), and a nested tree structure. The nested tree structure will not work with the Galaxy API currently - but it is the way the API should work :).
Twill-less tool tests fix for nested input filenames.
Twill-less tool tests fix for tests with html outputs.

For example gatk_analyze_covariates.
Twill-less tool tests fix for binary data in results.

Use requests' response.content instead of response.text for reading results.

Fixes the following error 'UnicodeEncodeError: 'ascii' codec can't encode characters in position 1-3: ordinal not in range(128)' when dealing with ASN.1 binary files.