Ignore Invalid requirement files

[TG1999]

Python-inspector doesn't support taking input from setup.py, setup.cfg for now. Raise an error when a file that doesn't match requirements file pattern is given in input.

[pombredanne]

@TG1999 IMHO we should NOT raise an error if the file does match a name pattern... rather we should raise an error only when it cannot be parsed.

[TG1999]

This #56 helps in not ignoring file on basis of their name but does not close this issue, we still need to define a way to ignore files that are not requirement files.