Prevent sigve from leaking file descriptors.

After using open() function it is important to always close the file
handler. There are 3 spots in sigver.py where files were opened and
never closed. Following commit introduces new function 'read_file' that
utilized 'with' keyword, hence ensuring the file will always be closed.
Merge pull request #139 from zaccone/fix-leaking-descriptors

Prevent sigve from leaking file descriptors.