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:

• https://github.com/Trax-air/swagger-parser/issues/40
• https://github.com/Trax-air/swagger-parser/issues/41
• https://github.com/Trax-air/swagger-parser/issues/42
• https://github.com/Trax-air/swagger-parser/issues/43
• https://github.com/Trax-air/swagger-parser/issues/44
• https://github.com/Trax-air/swagger-parser/issues/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!