add file parser tests and fixes for #40, #41, #42, #43, #44, #45

[mtherieau]

Have added several test swagger files, each of which triggers errors in swagger_parser.

Adds tests and fixes for:

• AttributeError exception in _get_example_from_properties() when object has no properties field #40
• KeyError exception in _example_from_complex_def() when inner object has no schema field #41
• AttributeError exception in _example_from_array_spec() when array items references simple type defintion #42
• TypeError exception in get_example_from_prop_spec() when property has null type #43
• AttributeError exception in _example_from_array_spec() when items specified as array of types #44
• TypeError in get_example_from_prop_spec() when type field is specified as a list #45

[mtherieau]

I believe the python2.6 travis failures are unrelated to this PR https://travis-ci.org/Trax-air/swagger-parser/jobs/214476975

Any comments on this PR and/or the issues it addresses? (or the accompanying tests?)

[cyprieng]

Sorry for the delay and thanks for the good job ! The failure on 2.6 is effectively not linked with your PR

[mtherieau]

thanks for merging this!